I hope you either keep your power or have fun practicing harmonium in the dark!
While my characters don't have access to electricity, anyway, I'll pretend all the lanterns/torches/other sources of light just went out and they don't have any way to light them again (somehow).
Volshebniks can train their ability to see better in the dark, but a lot of them don't since they're rich and spoiled and don't spend a lot of time in the dark wilderness.
Iyu, having never even considered spending his time studying this, would decide that it is of the utmost importance that he learn how to see in the dark right now. He might actually pull it off, but he would annoy everyone around him doing it and find a way to commit several acts of heresy in the process.
Sanya probably already knows how to do it because he's the type of guy who studies in his spare time. He would tell Iyu to pray about it, but Iyu wouldn't listen despite the fact that Sanya is better than he is at everything and he knows it.
Dasha would try to bully everyone into blindly hunting for the source of the blackout so they could beat that person up.
Semchik would assume that Iyu was the source of the blackout, but it's a tossup as to whether he'd share that suspicion with Dasha or just see it as an excellent excuse to go to bed early and avoid having to deal with everyone else entirely.
The knyazes would immediately start plotting how they could use this to drum up hatred against an outgroup in order to expand their territory and resources.
