First of all, the wedding couple or their parents were responsible for taking care of their guests where it came to this being a health hazard for elderly guests. Whatever was happening at the venue, it wasn't as if the doors had been nailed shut. there is Zero excuse for that, and waiting for a generator isn't enough.
You likely have a civil suit for the loss of services, but withholding payment may result in theft of services and fraud charges for whomever is paying. You need to sue the venue in court for what you didn't get. It won't be the whole amount, nor can you sue for medical costs for other people who should have been helped out of there.
Yes, they should have had or supplied a generator. But that doesn't mean the people holding the wedding had no responsibility to their guests. People got there- they could also leave. And expecting everyone to sit and wait around was arguably just as wrong as the owners of the venue not being prepared. Go speak to an attorney. You're not going to solve this here.