Ask for an explanation. The issuer may close an account because of inactivity, you no longer fit its business model, you've become too much of a risk or you're no longer profitable.
Fight to get your previous terms reinstated. If you've had a sporadic pay history, are near your credit limit or rarely use the card, you may not have a leg to stand on. But if you've been a good customer, it's worth it to call the issuer and plead your case. Have your financial ducks in a row before picking up the phone.
Prove that you're worth keeping. If you have a solid credit report and high credit score, you should be just the kind of customer any issuer wants.
Make them feel secure. If applicable, point out that you're in a field that is not susceptible to layoffs and that you have a steady income.
Be prepared to negotiate. Know what you want before you call. For example, if your interest rate has been raised and your credit limit has been lowered, start off asking that both be returned to the previous levels. But figure out beforehand what is most important. Do you need a low rate because you carry a balance from month to month, or does a high line of credit matter more to you?
In a standoff, escalate. If you're not getting the answers you want, ask for a supervisor. Move up the ladder until you either get what you're after or are convinced they are going to stand firm with their decision.
Inquire about an opt-out clause. If it makes more financial sense to do so, ask to have your account closed, with you continuing to pay the balance under the former terms. If it's going to be a true financial hardship to meet the new terms, then it's better to close the account.