I mean, it doesn't need to be casino's fault, but not even yours. There are 2 other subjects at least. Your bank and the payment provider that sends to money to the casino. The thing is you need to figure out who did the mistake there and the casino should always be willing to cooperate in this matter.
Anyway, you can start with your bank, tell them the money didn't reach the receiver and let them investigate. If their attempt to send the money failed, they should refund you or they'll get a proof that the money was send. It that case, you can forward the proof to the casino and then they should talk to their payment provider. All the transactions have their own transaction ID so they should be able to find the payment.
If it's a good casino, then it should be enough to send them for example a screenshot from your internet banking showing the transaction in order to start the investigation on their side. But you can always start with your own bank. The money can't get lost.
