The door lock fob works, but the key will not turn. What's wrong?

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Awhile ago I noticed that the door lock key would intermittently not work. The key fob as always worked. When the key would work, there was no resistance or acted anything but normal, however when the key would not work, it would not turn at all. This went on for some months. It seemed like temperature made a difference. More often it would work when it was warm, but not cold. Now it is completely dead, unless on rare occasion on a warm day it decides to work. What is wrong. Is it the actuator? Door switch module? Something else? Solonoid? It appears to be an electrical problem.