Your actions depend on what exactly went wrong with your deposit and whether you know the reason for the delay/decline or not. In most cases, you just have to wait for a few more hours/days until the deposit is processed. If the problem is not solved, contact:
a) the customer support service of the casino via email, telephone, or live chat if the casino runs one;
b) the customer support of the payment provider if you believe that that the issue is on its side.