I have a model year 2000 Murray lawn tractor, 14.5 hp, 40" cut - It always ran perfectly, but just a couple days ago, I cut my yard & shut the engine off & parked it - then I went out to start it and move it later - the dog gone thing starts fine, (no problem with the engine at all), but when I try to shift into gear (it has 5 gears & reverse), and also if I try to engage the blade clutch handle too, it will not budge - the tractor seems to be "stuck" and it makes the engine shut off due to all the resistance when trying to move forward in gear, and the same thing if I engage the blade - it stalls out right away. The motor is fine. It just can't move the tractor. Like something has either come loose, jammed, or seized up all of a sudden - Anyone had this happen or know what could be the cause? I cannot even move the tractor - I put it in neutral and tried to push it but it won't budge either.
Thanks for your help.

Sounds like a belt issue, Inspect the drive belt which is located under the machine belly, you may have thrown the belt or it got tangled somehow.