Check the expiration date on the front of the card to make sure the card hasn't expired. It will show a month and year in the lower right corner. If that time period hasn't arrived yet, your card should not be expired.
Locate the number of the credit card company on the back of your credit card. It will be a toll-free number that you can call 24 hours a day.
Call the number and follow the automated menu instructions to speak to an operator. Ask the operator if your card is still active. He should be able to tell you if the card is working properly or if anything unusual is going on.
Make a small purchase either online or at a store that takes credit cards. If the charge goes through, your card is working fine. If it is declined, you will need to get the card reactivated.