Im no mechanic but I have had a few problems over the years with garage doors. Firstly you need to release the door from the overhead track you should have a cord hanging down and by pulling this should disconect the door and allow you to manually lift the door. By doing this you will eliminate the posibility that there is something jamming the track or that it has for some reason jumped out of time. Try it up and down a few times be carefull coming down its heavy. You may also want to check that theres nothing breaking the lite on the way up like weather stripping. Hope these help

Take off the cover and look at the white nylon drive gear just in front of the motor. Odds are it is stripped and needs to be replaced.