Here’s the problem as I see it.
A NAS device doesn’t connect to your network thru it’s USB port. That port is to connect an additional USB type device to the NAS, i.e. another drive or a printer. So, even though it seems like something changed when you connected via USB, it really had no effect.
You are still accessing the NAS thru Ethernet. I think you have either a bad port on your dumb switch or (more likely) a bad Ethernet cable. If worse comes to worse, the return from the power outage cooked your NAS.
Before you buy a new NAS, try a different cable or take the switch out of the equation.
Hope your drives are Ok.
Unasked for advice - buy a decent surge suppressor and avoid future problems of this type.