The motor isn't as strong, the tracks are dry and full of chit, and the glass may no longer track straight. I'd push up on the glass (just a bit of pressure, don't force anything) while opening to full tilt. Opening ensures both sides have extended fully, and lessens the chance of it cocking to one side while closing. If it works, and it were me, once closed I'd never try to open it again. I wouldn't even look at it unless I planned to remove/clean/lube/rebuild as needed....