I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear that this happened.
1. I'm afraid you'd have to ask them that. There's no way for a third party to know.
2. Yes, they can. A business owner has a right to refuse to do business with anyone they like, as long as the refusal isn't based on illegal discrimination. From what you're saying that doesn't appear to be the case - but if it was, your recourse would then to be go after the venue.
3. Unfortunately, yes. You signed a contract with the bride and groom believing that you would be allowed to enter the venue and provide the service they were paying for. This is considered a mutual mistake of fact - had all the parties known about this before signing the agreement, you wouldn't have done it. In this scenario, the contract fails because it cannot be performed, and the law says that the parties have to be put in the same position they'd be in if the contract had never been signed. That unfortunately means that you have to refund the deposit. The non-refundable language in the contract does not control, because you physically cannot provide the services you agreed to. That language protects you when someone cancels for no reason, but it's doesn't allow you to keep payment when you don't provide any services.
If you can prove that the bride changed her mind and colluded with the venue to get you banned just so they wouldn't have to pay you, that would be a different story. But based solely on the information you've provided, you do not have a legal right to keep the deposit. I'm sorry.
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