It could be that your Xbox Credentials were corrupted in the power outage. You can delete them and restart your computer, when they'll be regenerated automatically.
To do this, type Credential into the taskbar search window.
Click on Credential Manager.
Click on Windows Credentials.
Click on each entry beginning with XblGrts and select Remove - there are a lot of them, but it beats a full re-install!
Close Credential Manager and reboot your PC.