Several things can cause your key not to turn when inserted into the car door. You can get into your car in most cases regardless of the cause, but it may require you to try a few different methods to gain access. Replacing the locks on your car may be necessary if you cannot get the key to work otherwise.

Frozen Lock

During the winter you may not be able to turn your key in the car door because moisture has gotten into the keyhole and frozen. Thawing the lock so the key will turn freely is simple. Buy a de-icer spray from an auto parts store and spray it into the lock and onto the key. In many cases this will be enough to loosen the ice and allow you to unlock the door. According to CarAndTruckTips.com, you can use a cigarette lighter or match to heat the end of your key. Be careful not to burn your fingers as the metal will conduct the heat and may make the end of the key where you are holding it hot as well. Insert the key into the hole and allow the heat to melt the ice long enough for you to unlock the door. If you have a power outlet nearby, use a hair dryer set to high heat to blow on the lock. This should clear up the problem within a few minutes and may dry out the moisture trapped inside to prevent refreezing.

Lubricate Lock

If dirt or some other residue has contaminated your car door lock, it may prevent the lock from turning when you insert the key. Spray a lubricant such as WD-40 into the lock until the liquid lubricant runs out the side. Insert the key and wiggle it gently back and forth to help spread the lubricant around.

Slim Jim

The slim jim is a device used by police and locksmiths to open car doors when someone locks the keys inside. The user slides the device down past the windowpane to access the locking mechanism and manually pulls the lock open. Be aware when using this method that it can sometimes damage the car permanently. Slim jims and other objects used to forcibly unlock car doors are not as effective on newer model cars because the cars are designed more securely, according to StreetDirectory.com.

Keyless Entry

If you have a car that has a keyless entry "keypod" that opens your locks with the push of a button, then the fact that your key won't turn is not an emergency. Simply press the unlock button and open the door as usual. Even if you have keyless entry, you should get the problem with your key fixed as the battery in your keyless entry remote may die at any time.

Source:

CarandTruckTips.com: Frozen Door Locks

Street Directory: What Are Your Options When Locked Out Of Your Car?