What happens when your credit card expires?
You cannot use it until you renew it.
Generally, the bank will renew credit cards automatically and send you a new one in the mail as the old one expires. You will probably have to call in and activate the new card before you can use it, then make sure it works and shred the old one. In the extremely unlikely event that they don't renew it (terrible credit, etc), they will send you a letter stating why.