Why did your wifi went down ?
Some wifi and zigbee are on the same wavelength, if someone was jamming it (this is illegal), both can be taken down like this.
How is your coordinator linked to your home assistant?
For example if you use Zigbee2MQTT and you have either Zigbee2MQTT or Homeassistant pointing to the internal ip of mosquito (192.168.1.11 for example) and your router goes down (with dhcp), it's possible it cannot communicate anymore.
This isn't the case if it's all running on the same box using localhost as address, running it in a docker network or when you run ZHA however.
that uses mDNS, which in some cases requires your router to be online to be able to resolve it to a ip. If part of your internet disruption was your router going down, it would explain the issue
Sounds like from your comments that you mean internet.
Zigbee does not rely on internet, but does rely on a local connection to your mqtt server which may be on another machine (ideally not), and if these machines are connected via wifi (ideally not) and your wifi went down, then yes you can lose functionality.
I had something similar (although probably unrelated) happen last week. All of a sudden my router went offline and nothing was getting an internet connection. Sonoff coordinator lost connection with all zigbee devices went unavailable at the same time. Tried rebooting my home assistant pc, router, unplugging sonoff antenna, switching Ethernet ports, etc without luck.
Power cycling my Ethernet switches fixed it immediately. No idea why that did the trick. This has never happened before. I’m on Verizon fios if that matters.
Posting here in case it helps someone identify a common thread/root cause for OP and me, because I thought it was bizarre that an Ethernet switch would ever need to be power cycled!