Sometimes the electrics fail and you're stuck.  Here's a vaguely popular group of people that cavers may have heard of and how it was dealt with. Ironic that the song title was what they were doing before it was fixed.

https://www.youtube.com/watch?v=3BDo8m8UoE0

If the power is cut, you can always play outside the venue like Seth  Lakeman did once. Seth Lakeman and band play Blood Upon Copper outside Bristol Academy after a power cut forced the early curtailment of their gig:

https://www.youtube.com/watch?v=hTvfPWG6OLs