Try this: Takes 2 people. From the drivers switch, hold the window button down and have someone slam the door. If it rolls down, could be the motor or a bad connection. If it doesn't work, repeat the process using the rear passenger seat.
Sometimes as they get old, the window motors will develop a dead spot. The "push n slam" method might free it up again...but it's a temporary fix. My 89 K2500 started this on the driver's side about 3 years ago, and I've got to do it every now and again. Hope that's all it is.
GMCYuke, the second thread is a nice find. This is the stuff that belongs in a Tech Sticky!