My 855 is essentially the same tractor. Here's what I would do.
First - Try the stuff already listed. Make sure the parking brake is off. Put it on, take it off, make sure it is working.
Make sure the range selector isn't in neutral, try it in high and in low.
Make sure you are sitting on the seat, as it controls the safety switch already mentioned.
All the obvious stuff.
Then if this doesn't work, look at your Hydro pedal linkages. Do they move or not. If you follow the linkage back, the pedals connect to a lever on the actual hydro tranny. On mine, I had the bolt break and fall out. So I could move the pedals, but the tractor wouldn't move. Replaced the bolt and things were fine. Nice thing about these tractors is by looking under them it's pretty obvious what's connected to what.
Still not working, use the outriggers to lift the tractor, put it in neutral, and try to spin the rear wheels by hand. Has to be in neutral or the hydro will hold the wheels. Spins - brakes are fine, Stuck - most likely brakes. Front wheels aren't a good indication, as the diff will allow them to move even if the trans is jammed. Then put it in 2wd, low, and use the trans to try to turn the raised wheels.
Should give you lots to start with. I'm assuming it worked fine when you parked it, and nothing "happened" that you noticed.
