I do not have any hardwired switches, apart from an aqara switch that controls my boiler.
Have restarted Z2M with no change!
I did re-install Z2M last night, I did take a backup of the Z2m config folder before uninstalling.
Could it by a mqtt issue now? Does Z2M need to log in to mqtt?
Also my HA backup from Sat am was my last know good.
KR Paul.
Don't think that's the issue because you can see Z2M publishing an empty payload to the device MQTT topics in your startup log. That's why you're not seeing any devices (empty payload will remove the device).
Since you have an older backup of the Z2M folder, can you compare that to the current Z2M folder?
You should be looking at 2 files:
"devices": section.You can use an online file comparison tool like https://www.diffchecker.com/ if you don't have a way of comparing files locally.
Configuartion.yaml is OK.
Coordinator is bare! No devices in it.
Can I copy my original coordinator file in to the config folder? Or copy the text from one to the other?
Thank you...
Yep. Stop Z2M, delete the newer backup file from the folder & replace it with the older copy, then start Z2M & keep an eye out on the logs
Did all that.
When I restart Z2M it cleans the coordinator_backup.
I thought I had put the wrong file in, so tried again, it keeps putting basic info into it, wiping everything that was in the original file I copied in.....
P.
I'm running out of ideas 
Might be time to try reflashing the firmware back to 3.3.1 if you haven't done so already.
Already at 3.3.1
Really do not understand, as you mention, how HA have released dev firmware to the public.
My only option now as I see it is to restore from my known good as of yesterday and hope fr the best.
Appreciate your help and everyone else's.
.... 2 years no glitch then one update screws it all up.
Had a spare Aqara H1 which I never fully commisioned.
When I joined it it knew its name.....
Can I just 'rejoin' everything and it should be OK?
Or
Do I restore my backup from Sat morning?
P.
HA didn't release that version - SMLight did. If you're on 3.3.1 because you reloaded a backup, then I suggest you re-flash the firmware directly from the SMlight integration or via this SMLIGHT FIRMWARE UPDATER
I only assumed it was HA as I thought updates could only be released by them.
Shutdown Z2M
Using Samba, Renamed the old zigbee2mqtt directory to old.....
Copied in the original working one.
A couple of reboots later and I have my devices now.
It picked up the Innr SP242's nigh on instantly.
I had to power off and on my upstairs and downstairs lighting circuits. I have various Zigbee devices on those circuits, like the Aqara CL lights and 2 channel switches.
Aqara heating and temp sensors are back online.
Z2M Log is now displaying blue entries and not red ones!
So glad I dod not have to relearn everything again.
Thanks @ShadowFist and all that suggested solutions.
Give SMLight a kicking for me / us please! 
Heh, funnily enough, I don't work for them. Don't even own any of their devices - I'm just a random guy trying to help.
I suggest you contact them & ask them to add a toggle to hide beta firmwares in their OS (and default it to on). Should be pretty easy for them to implement & would avoid headaches.
Remember to mark whichever post provided the answer with the solution tag. That way others with the same issue can find the answer without having to read the entire thread.
This worked for me.
The solution was everyones help. And the icing on the cake was using the original config directory from a known good backup.
Anyone else reading this thread, The solution was solved by using everyones input by @ShadowFist @mr-bembel @brad.trout and @jmateus72 suggestions.
Notes to take
Factory resetting the SLZB, Install 3.3.1 firmware using the devices webconsole
taking the aerial off, rebooting HA, wait for it to settle, aerial back on.
If you have No devices, then stop Z2M.
Using Samba, rename the zigbee2mqtt directory to oldzigbee2mqtt
Copy in the LAST WORKING zigbee2mqtt folder, Reboot HA System
Wait patiently
If your routers are lights, reboot the lighting circuits. If things are still missing reboot the endpoints too.
@phewson I experienced the exact same issue. Thank you everyone for your efforts in resolving this. Your instructions made it relatively easy to fix.
I did notice there has been an update for Z2M (2.11.0-1). I couldn't see anything in the change log about the SLZB-06. But I can report that there have been no issues with the update after fixing using your instructions.
Another person here got caught out getting the "dev" firmware installed. It happened around the same time as an HA core update and a Z2M update, so I didn't know what had knocked Z2M offline (it refused to start due to errors stating PAN_ID plus others already existed).
I rolled back HA Core and Z2M and managed to get Z2M to start, but like Paul, I now had no devices! It was after that I discovered the dev firmware, so have now rolled that back to v3.3.1 via the SMlight webconsole. Still no devices 
Looking in the config.yaml, the devices are listed, but in coordinator_backup.json, no devices are listed. I haven't got a "last working" zigbee2mqtt folder to revert back to. I've tried all the other suggestion above as far as I can including the "remove the ariel" incantation.
I'm running HA OS on a an HP Prodesk. Any other info I need to give that can aid getting me back up and running (without have to re-pair a couple of dozen of devices)?
Some screen shots:
Whilst it is showing 8 siblings on the network data map, there are 17 listed in the config.
Like a bad smell, this popped up in HA tonight. I've found the toggle to stop automatic updates in the SMLight web console, under Settings and Tools -> Firmware Update and turned them off.
did all the above, now zigbee2mqtt starts, but no devices 
first it would not start, i rolled back the firmware and reflashed the controller, then the controller started with its own pan-id and channel, i set that back through the webinterface, still conflicts because there were two conflicting networks (i read in the logs), i did the antenna trick with a piece of aluminium foil over the SMA connector - then the app started, but without devices.
in the file "coordinator_backup.json" I can see the list devices. I tried rolling back a backup of home assistant zigbee2mqtt but i see that some of the files stay to be from today. I have no idea where I could get the whole "older" directory. I leave it to home assistant to make good backups...
any tips? would be a nightmare if i need to re-pair all my 50 devices...
update; it struck me that although i rolled back a backup of zigbee2mqtt through the "normal way" of home assistant, the file "database.db" was always recent and empty exept for a single line.
I stopped the zigbee2mqtt app, copied from a backup of 2 days ago the file "database.db" over the existing one in the path homeassistant/zigbee2mqtt/... and started the application again..
whop, my whole list of devices was back!!! some are not "online" yet, but the vast majority is. pfffffffff.
SMLIGHT, you made a big f#ckup - that cost me over half a day!!! 
thanks for all the ideas. so this is one to add to the list of solutions; copy back an old database file.
I tried copying a backup of my .db file which appeared to have data in it, several times. Each time I restart Z2M, the db gets overwritten and just has the coordinator and groups left in it.
The official SMLIGHT ticket:
https://support-legacy.smlight.tech/supportcenter/public/en/blog/zdo_statechangeind
