1. TRY ANOTHER DOOR: It sounds simple, but many people get flustered and panic when they discover that their car door lock is frozen and they may not think of trying another door! Try another door or the hatch. Often, one of the other door locks will work just fine!

2. USE THE CAR DOOR LOCK DE-ICER: If you live in a cold climate, spend a few dollars and purchase a little aerosol canister of lock de-icer. Lock de-icer is easy to use and it can be used to unfreeze a car door lock or a frozen door lock on your home, office or other building. Insert the door lock de-icer nozzle into the door lock, squirt and unlock the car door.

3. WARM YOUR KEY WITH A LIGHTER: If you don't have lock de-icer handy, move on to this step. Use a lighter or matches to warm the key for a few seconds and insert it into the lock. Sometimes, this must be repeated several times to completely unfreeze and thaw the car door lock. Be careful - the key is hot!

4. WARM THE DOOR LOCK WITH A HAIR DRYER: This is the most time-consuming method for de-icing a car door lock. You'll need to run an extension cord out to your car and focus the hair dryer on the door lock to unfreeze the lock mechanism.

 I have heard of some using Petroleum Jelly on the key also, but I have never tried this and would be hesitant to do so, as it may deteriorate the internal parts of the lock over time.  The same goes for WD-40 or any type of petroleum based chemical.