Which model router?

Same problem using a Valerie modem from Virgin. 

Xfinity XB6-A. The Sonos speakers worked for two months, but not after a recent power failure

Refrain from any SONOS Factory Reset without further consult.

During the power failure and restore there can be dangerous transients that result in hardware failure. We cannot rule out hardware failure at this point.

As the power comes back, there can be clumsy interruptions that will fluster the network clients and routers. As first step, I recommend shutting down all network clients, restarting the router, wait a few minutes, then start network switches (if any), then network hard drives (if any), SONOS units, then everything else. If you have a surround Room, restart the soundbar first.