The physical reset they mentioned above helps in a lot of cases, but before you do that try something simpler first: check if your router made it through the power outage okay. Sometimes the problem isn't the blind but your wifi got messed up after the power cut. Completely restart your router (unplug it for 30 seconds), wait for it to come back up properly, and only then try to reconnect your Somfy from the app. If your phone connects to wifi normally but the blind still doesn't show up, then we move on to the next step.
If the router is fine but the blind doesn't appear in the app, unplug the blind from power for at least a minute (not 30 seconds, longer). Some wifi modules need that deeper reset after losing power suddenly. When you plug it back in, open the app and keep an eye out to see if it shows up as a new device or in pairing mode. The app should give you an option to reconnect it like it's the first time.
If after all that it still won't show up in the app, the wifi module probably got damaged in the power outage. Then it's worth calling a technician because you'll likely need the module replaced, it's not something you can fix at home. But try those steps first because they work in most cases.