As this is a router, there is probably a modem sitting in front of it. Power cuts usually take out both the modem and the router on a network. When the power comes back on, they may start up in the wrong order, with the router starting before the modem is ready. That means that they get confused about which is in charge.
Be sure to restart your network in this sequence:
Turn off and unplug modem.
Turn off router and computers.
Plug in and turn on modem. Wait 2 minutes for it to connect.
Turn on the router and wait 2 minutes for it to connect.
What else have you tried but can't be bothered to tell people?
Your first message provides next to no useful information.
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