Passenger Side Door Not Locking

Yesterday evening, I noticed that my passenger side door of my 2006 Prius was not locking. When I lock the car and try to open the passenger side door without the fob, the alarm sounds, but the door opens. I tried locking it manually from the inside with both the button on the armrest and the door and the same thing happens. Both fobs are out of the car and the car is off. The keyless button on the passenger door seems to be working but the door doesn't actually lock. Nothing seems to lock it. All other doors are locked. The car is still under warranty. The car was recently returned to me after an accident, but thinking back, I remember my son opening the door and the alarm going off back in Nov. I am pretty sure the car was locked then, so it might not be accident related. I have an appointment with the service department on Monday morning. Any ideas would be greatly appreciated. Thanks!

 

Thanks so much. I like to have a clue as to what I'm talking about when I take it in. One more detail is that it (the front passenger) door doesn't lock at all, even when the car is running and even though it looks locked.