Transmission 1992 300E

I went to the grocery store today and when I got back to my car and tried to back out, my car would not move. It seemed like something was holding it back. At first I thought I had forgotten to release the brake but that was not the case. I sat in the parking lot for about an hour, hoping the person in front of me would come so I could just drive out. I eventually had a couple of guys come and push me back where i go turn the wheel and get my car out of the parking space. My car runs fine in Drive. You would not know there was a problem. I don't want to take my car to the dealer and be told I need a new transmission when it might be something less expensive. Have anyone experienced this or does anyone have a clue as to what might be causing this.