Loud bang when opening front passenger door

I have a 91 Lincoln Town Car Executive series, and I went to school without the remote or door key, and I locked the doors. A security guard used a slim jim on the front passenger door unsuccessfully, then tried it on the rear passenger door, which he opened. Now, when I open the front passenger door, there is a loud bang coming from inside the door.

A couple of times it was hard to open; opening only about 1/3 of the way. I had to close the door, and try to reopen it until it didn't stick.

What could be causing the problem?

Check the clearance between the door and the fender. The door is likely "pinching" against the fender as the door is opened.