pop-up sunroof won't pop-up

[Boston Benz]

RE: a 1992 300TE. The sunroof on this car slides back & forth beautifully. When the button is pressed upwards, the roof is supposed to open about an inch or two for ventilation. Mine's not doing that at all-the motor whirrs as if opening it but it's not. If you push up on the back of the sunroof while pressing the button, it will go up. It closes normally the way it should. While pressing the button up for the ventilation option, you can hear the motor running. It sounds like something popped off its track.

Has this happened to anyone before? Any quick fixes to this?