Shelly has released a new firmware v1.10 on 18.03.2021
Beware! After installing the new firmware on my shelly 3EM, all Shelly sensors became unavailable in HA!
This link can help you to downgrade to version 1.9.0 if like me you upgraded too soon 
https://www.shelly-support.eu/index.php?shelly-firmware-archive/
link to Shelly forum : https://www.shelly-support.eu/forum/index.php?thread/8666-shelly-firmware-1-10-released/&pageNo=2#post94560
What version of HA do you have?
And what integration?
I doubt mqtt integration is affected.
The original message could be more precise.
System Health
version: core-2021.3.4
installation_type: Home Assistant OS
dev: false
hassio: true
docker: true
virtualenv: false
python_version: 3.8.7
os_name: Linux
os_version: 5.4.83-v7
arch: armv7l
timezone: Europe/Zurich
logged_in: false
can_reach_cert_server: ok
can_reach_cloud_auth: ok
can_reach_cloud: ok
host_os: Home Assistant OS 5.12
update_channel: stable
supervisor_version: supervisor-2021.03.6
docker_version: 19.03.13
disk_total: 13.9 GB
disk_used: 2.1 GB
healthy: true
supported: true
board: rpi3
supervisor_api: ok
version_api: ok
installed_addons: File editor (5.2.0), Samba share (9.3.1)
mode: storage
with core shelly integration
Works without issue on a Shelly EM and 2.5
I have the same problem with the Shelly 3EM. Updated on the afternoon and HA sensors became unavailable when I logged in the next time
I deleted the device and try t to add it again but it always says connection failed.
But it talks to the device, I watched it with tcpdump.
All sensor values are send to HA and that says connection failed
are you using native shelly integration?
Then please state it in the subject and issue description
I’m using Shelly-Plugs and Shelly 2.5, updated all my devices to 1.10 and even rebooted my HASS, after having read the warning. I’m not having any issues at all. I’m running the native shelly integration.
My HASS is core-2021.3.4, supervisor-2021.03.6 on Home Assistant OS 5.12.
It seems to be only a Problem with 3EM then…
There’s already 2 new versions the
20210320-162133/v1.10.0-em3-g495c2e7
20210319-202425/v1.10.0-sw25-g1f21203 that replaces the
20210318-140154/v1.10.0-geba262d (version that generated the unavailable sensor by me)
Didn’t had time yet to give them a try, but i’ll wait a couple days like there is actually a new version everyday…
Annonce from Shelly support group about the last version:
Shelly3EM - 1.10-3EM released, fix compatibly issue with Home Assistant and other CoAP based integrations.
Shelly2.5 - GPIO set which cause high CPU working temperature - fixed
ShellyI3 - going in factory mode after reset - fixed
Shelly Dimmer1 - cannot be controlled after reboot - fixed
I can say at least that with the Shelly Button, wake to wifi connect is substantially faster, went from around 7s to 4s, so their note about faster connection of battery devices is legit
Firmware version
20210320-162133/v1.10.0-em3-g495c2e7
on Shelly 3EM successfully tested with HA native Shelly integration
I can confirm that