Almost everything is ”not available”

Hello, I am a newbie in the field of Home Assistant. I have been using it for about 10 days. I really miss a handy step-by-step guide. I used a recent article in iCreate magazine to make the first steps but missed all kinds of details. Anyway, after I installed HA (HA Green with ConBee III Zigbee USB Dongle), I created my own dashboard with different tabs for lights/sockets and media. Everything seemed to work fine until suddenly nothing seems to work anymore. Lights (IKEA and TUYA) “not available”, sockets “not available”, printer “not available” etc. What could be wrong?

Start by looking at the error and warning messages in Settings → System → Logs.

If Home Assistant loses its connection to the Conbee III dongle, all Zigbee devices will become unavailable.

Conbee is like pre-2020 technology. There are a lot newer things I would suggest instead of that.
If you want an index of Tips from the Home Assistant user community, look here:

The Home Assistant Cookbook - Index