I experienced that today - late at night, I was unable to control all lights (I have 116 devices), and my 2 Hue bridges (Inside Lights and Outside Lights) all showed this:
Update 5 days later from the original issue occurred - One Hue brigde (Outside Lights) has all bulbs and sensors as unreachable again, the other (Inside Lights) is fine.
Tried to power of the bridge and the power to some bulbs, with absolutely no effect. Even almost stranger, I can see that the sensors on the brige became unavailable in Home Assistant over a period of time, its not like just - all unavailable suddenly at the same time ...
I have disabled Software updates for both bridges.
The bridges seemed fine, and I (beside panic) started to investigate:
Cursing...
Yes, I did that
Googling
https://www.trustedreviews.com/how-to/fix-philips-hue-lights-unreachable-error-3631990
https://www.online-tech-tips.com/smart-home/philips-hue-lights-unreachable-7-things-to-try/
Power cycle bridges
Did nothing at all
Rebooting network Router
Did nothing at all
Restore a Bridge
I use http://www.all4hue.com/ to take backup of the Bridges.
I just got some double entries, and the restore seems to be ment for a factory resat bridge.
I tried the above a few times, and nothing changed.
After aprox. 1,5 hours I took the power to the entire house, and after the bridges came up, the Outside Lights bridge had most bulbs available again, but many sensors were not reachable.
Be sure to set Your bulbs to not turn on after powercycle This is the Power-on behavior setting - You need to do it for each bulb...
So I went to work for 8 hours..
When I came home, I took the power to the entire house again, and after the bridges came up, aprox. 80% of the bulbs on the Inside Lights bridge were reachable - and all sensors seemed ok again.
In the evening I started to power cycle the power sources for the bulbs that was still unreachable, and slowly they came back in the app as reachable.
The next day, I took the power to the entire house again, and 3 bulbs were still not reachable. I took them out of the ceiling sockets and put them into a lamp, one by one - 2 on them actually came to life again and the last just had to be reset (power off 5 seconds, power on for 5 seconds, do that 5 times - when the bulb flashes, its resat).
So all is good now :-)
My conclusion is somewhat, that this problably was a software update - or similar - that kind of "broke" the bulbs ability to communicate - and software updates over Zigbee is slow and takes a long time.
Point being: Dont panic to soon and start resetting your bridge(s), have some patience and take the power off some times over a time period. Setting up everything again can take way longer time if You have many devices.