It's possible someone tried to jimmy your lock or punch it out. Does anything around the key cylinder look damaged?
If not, you might buy a tube of graphite lock lube and follow directions--or some other lubricant made especially for locks. Could be that one tumbler just jammed.
If all that fails your only other choice would be to remove the door panel, unbolt the lock from the door, disconnect the locking lever from the inside door handle, and take the lock to a locksmith for a replacement cylinder---or buy a new cylinder and key and insert it into the lock itself.