I have a Craftsman 1/2 HP garage door opener. It's always been a little finicky, but tonight it stopped working. In the middle of going up as tonight, the door halted, leaving a gap between the door and the ground. I got out of the car, squeezed under the door and realized the motor was still running. My wife pushed the door opener a few more times, but the door wouldn't move despite the motor responding. I pulled the release on the door and opened it all the way. I realized parking the car in the garage would be a hassle, since I would have the open the door manually again. So I just closed the garage with the chain disengaged.
The motor will start and stop in response to button pushes like it is supposed to - normally the first push starts it in one direction, the 2nd reverses the direction and the 3rd will stop the motor. However, the chain has completely stopped moving (and by extension, so has the door). This causes the motor to run for an extensive period of time before it seems to timeout and give up. The chain and door connector assembly haven't budged since.
I've not worked on garage doors before, but I have a degree in Computer/Electrical Engineering. I've never studied to be an electrician or anything, so I'm well aware there are a lot of things I don't know about this. But generally speaking, is this the type of thing I could potentially fix myself? Or is it more likely the whole unit is shot and I should just call a repairman up front?
I know I have the manual around here somewhere, I'll keep looking for it and list exact part numbers if I find it.
Update: I popped the sprocket protector off just like in the video form Steven's answer and the chain is still firmly attached. When the motor is running, you can clearly see the shaft and sprocket are not moving at all, not even jerking when starting or stopping, like something had locked up the shaft.