2000 Cadillac Fleetwood Driver side door

What about trunk cylinder. There is a possibility that the ignition was re-coded and the door cylinder was not changed. How long have you owned the car? If you have owned it since new, then you would know if a procedure like that has been performed. If it is known that it has not, you would have to disconnect the linkage rod from the lock cylinder and see if it turns, if it still don't, there is an issue with the cylinder like a stuck tumbler or other fault.