Transmission Issue or Clutch?

Just took mower out for first cut of season. It was running fine. Got to small hill and changed gears and it wouldn't move. Pushed it to level ground to try and engage again but it wouldn't move.

First thing to check would be the drive belt tucked up inside the frame. make sure it hasn't broken or come off any of the pulleys. If that is not what it is, then post back with the the model number of the tractor so we can see what you are working on.