Keeping your car doors locked is one important safeguard against auto theft and vandalism, but it can also backfire. For any of a variety of reasons, occasionally car keys won't turn in the door. There are a few reasons this can happen, but they are all solvable problems.
One step is to call for professional help. A locksmith or police officer will likely use a device called a slim jim which slides inside the window pane and unlocks the door manually. This is a quick fix and will work particularly well on older vehicles, but there will be a fee associated with this service if a locksmith is used.
Thawing the Lock
In colder areas, a lock will sometimes become uncooperative if it is frozen. It can be thawed using a product called de-icer, which will contain simple instructions along with it. A blowdryer can also be used on a lock if de-icer isn't available.
An old lock might just need some lubrication in order to turn properly. In this event, use some professional lubricant like W-D 40 to get the gears sliding better, then try the key again. If the problem was just debris or rust, lubricant will solve it.
Most newer automobiles also contain keyless entry. If your car has a keyless entry system, the key not turning won't be a problem. Simply unlock your car using this separate device.