How big is the dent? Unfortunately, door dings happen all the time. My passenger side door is proof of that.
The UCF parking lot is filled with idiots who are physically incapable of opening their doors without smacking into the car next to them.
If the dent isn't too big, you can buy one of those dent kits online for about $20 to $40. It does a pretty good job of removing door dings.