First, is the key able to turn the lock at all ? in other words, is the key working the lock it self ? Can you turn the key at all ? You will need to remove the door panel to access the lock, and rods that work the lock and key cylinder, the rods, and the door latch. With the door panel removed, you can look and see if any of the linkage rods are like they should be, and in good working order. You can remove the linkage from the key cylinder, and see if the issue is in the key cylinder, or some where else. Sometimes, the rods and rod holders get corroded, and need to be cleaned, also sometimes the rod holders brake off and need to be replaced. Then there is the door latch lock itself that can become stiff from not using the lock. A little penetrating oil spray, can clean up old stiff grease.