That black stripe on the back of a credit card is a magnetic strip, often called a magstripe, formed of iron-based particles that somehow organize themselves to store and communicate information. When you swipe your card, information about the transaction and the account are sent electronically to a company for verification. If everything checks out, your purchase is approved and you are on your way.
Damage to the data strip on the back of your card can make it impossible to swipe and pay. There are a lot of funny theories out there about what demagnetizes a credit card. For example, Snopes debunked the myth that eel skin wallets will damage a credit card strip. There are also people who are worried that their duct tape wallets will demagnetize the strip. But for the most part, credit cards are pretty resilient. For example, I know from experience that freezing a credit card isn’t a problem.
As with most things, hitting a credit card with a hammer or melting it in the microwave (yes, people have tried both) does cause damage. But the most common way a credit card strip gets damaged is when it gets too close to a magnet. Magnets can rearrange the iron particles making the information unreadable. Purses and wallets with magnetic closures are often the culprits. Placing one credit card strip directly on another credit card strip can also cause damage. Another common way for a strip to get damaged is from plain old overuse.
If your credit card suddenly stops working, you can ask the cashier to manually enter your account information. Then, you can contact your lender and ask for a replacement.