At 5:20 AM (EST) yesterday morning a WebmasterWorld thread reported that valid credit cards were being rejected and denied by Google. Meaning those advertisers who had depleted budgets were unable to refund their accounts, and get their ads back online. Five hours later, AdWordsAdvisor posted that the issue has been resolved.
Early word has it that the issue with the processor has now been resolved, and re-entering your credit card at this point should get you running again.
Please note that a delay between the time one re-enters the card and the time the ads begin running again is normal - and will be the case here as well. So once you've re-entered your card, if you don't see your ads running immediately, there is no need to enter the card again. I'd say give it an hour - though it'll probably be quicker than that.
Through this fairly short period, AdWordsAdvisor was there providing insight into the issue.
Forum discussion at WebmasterWorld.