1) The door did not open when you tried to open it. 2) Perhaps the door was locked and that is the reason it wouldn't open.

According to my friend, the difference between these two sentences is the time they are happening in. The first sentence denotes to the past while the second one occurs in the present time. You couldn't open the door in the first sentence. Yet, in the second sentence, you are saying that you can't open it. Make sense?