Sounds like the electric clutch that engages the mower deck operation has failed! But, before undertaking any large forms of repair--check the wires associated with the operation of the mower deck, etc. It is possible that a wire has come loose, or fallen off, or broken.
Those electric clutches aren't cheap, so do not attack the problem by first buying a new clutch. Check first for broken or detached wires under the tractor, and then else-where. RJ

Phil - I guess your search of the forum archives didn't turn this posting up, so here's the link to a fairly comprehensive primer on tracking down problems with STX PTO's. I see that Rusty thinks your PTO (clutch) has failed, but I think it's much more likely that you've got a wiring problem. Check out this link.

Here is a link that might be useful: Deere STX-38 PTO troubleshooting