newbie + 5310 = help me!

Guys, first post so please forgive me. My neighbor has a 5310 and he lets me use it as needed. Today I went to start it and it idled fine. When I put the tractor in gear, it would not move. I can feel the shifter go into gear just nothing happens when I let out on the clutch. I tried A, B, C and no changd. Where can I start to attempt to diagnose this problem. The tractor ran fine about two weeks ago. Any help is much appreciated.

Thank you in advance