Small Engines (Lawn Mowers, etc.): Moving Problem, chain and sprockets, shifting gears

Question
QUESTION: my john Deere 68 is having trouble. I put it in gear but will not move. Any ideas on what it could be?

ANSWER: Have you checked the drive belt?  Does it feel like it is shifting gears correctly?  Ddi the problem occur while mowing or has the mower sat a while?
Let me know.
Eric

---------- FOLLOW-UP ----------

QUESTION: I have checked the drive belt and it is on. And the problem happened when i was using it. It was working fine, then i parked it for a minute, but when i went to start it up and use it again it wouldn't move. It also feels like it's trying to go and it catches sometimes but rarely.

Answer
I believe you mower uses a chain from the gearbox/transmission to the rear axle.  Is the chain and sprockets in good shape.  If you look at the gearbox/transmission you should find a small metal plate with the gearbox model number on it.  Can you find the gearbox model number?
Eric