I hope that I am not the only one who has had the misfortune of having their trackball get stuck and not move in one direction... But I couldn't find any posts on it so here is a link that I found helpful in getting mine back to going.
I had the malfunctioning trackball, wouldn't go up. Blowing compressed air didn't help. I then popped off the silver rink using the procedure outlined in the link, and this lets the trackball unit out. I didn't disassemble the unit but just used compressed air on it and in the space, put it back together and -- fingers crossed -- it is working fine. This is a *much* less risky procedure than disassembling the trackball unit.