Locked room mystery

A locked room mystery is what happened the other night. Y'see, there was a lady found dead in her room, murdered. But, y'see, the door was locked, y'see, and only she had the key, y'see, and according to forensic evidence, the door wasn't forced, y'see. Also, there were no other modes of entrance or e...gress to or from the room respectfully. So, the question, as I rightly see fit to put it, must inevitably and inexorably be, "How did'e do it?"

(Scroll down for the answer)

The answer

The door wasn't locked/there was another means of entry/the door was forced/the killer had another key/he was hiding in the room when the police arrived/she killed herself.

Hey waitaminnit, those endings totally contradict the premise! This sucks. I'm calling my lawyer.