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!

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.