Kind of weird one here too.

We decided to replace the switches for our lights, upstairs.
This involved turning the power out, at the fuse box.
And then going with on/off cycles, several times as switches were changed.

The power went on and off, maybe half a dozen times over an hour or so.
I had internet without issue prior to this.
Then I try to log in, but have partial network access, but no internet.
Reset the modem, router and reboot the PC, no luck.
The secondary system has no internet either.

Connected directly to the modem, the PC works fine.
So it seems my router went.
Well, it is at least 10 years old, but has served me well.
Decided to see if the spare router worked (a wireless one).
And it did.

Not sure why the router would decide to go.
From powering on and off, several times for an hour.