If your car door will not unlock with the key, but the key is still working to start the car, then the problem is most likely with the lock on your car door rather than the key. Read on for potential problem areas.
One possibility is that the lock cylinder needs new tumblers. These could be replaced or you could replace the entire lock. You will need a locksmith that specializes in auto locks in order to do this.
Another reason that your lock may not be working is that the latch assembly is jammed. In order to check for this, you can take off the door trim panel and check the rods. If the rods are not the problem, you can then take off the lock. You will probably have to take the entire door handle assembly out in order to do this.
After you have taken the lock out, put your key in the lock (make sure you do this or the tumblers and springs will fall out) and then remove the retainer holding in the inner piece of the cylinder. There may be a lock cap that you will have to take off first.
Once you have taken the lock cylinder apart, check and see if any of the tumblers are not flush with the inner cylinder. File down any that are not. Put everything back together and see if your lock is now working.
If you are not able to perform these fixes or they do not work, then you may need to contact an auto repair professional or a locksmith. You can use TalkLocal to receive up to three phones calls in minutes from high quality auto repair professionals and/or locksmiths in your area that are available when you are.