Does your Express see other networks around you? If so, then the Roku is working fine. It's possible that when your power was restored that your WiFi router changed the channel it uses to one that Roku doesn't support. For the 2.4 GHz band (the only one supported by the Express) that channel must be from 1 to 11. If it's set on 12, 13 or 14, your Roku won't see it. You will need to manually assign the correct channel number.
If your Express is seeing your network, but simply won't connect, try resetting the Network Connection in your Roku settings menu. This deletes all saved network settings and passwords, so you can start with a fresh connection attempt.
Dan
Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.
