If you left it with the parking brake on then the brake can be rusted on, a lot more common than most think
The forward & reverse levers are not direct but work springs so the actual lever on the gear box could also be rusted in position.
As above, the belt could be broken, jumped off a pulley.
Find neural & dissengage the drive see if you can push the mower back & forth.
If not then brake is No1 suspect.
Jack up the rear and put it on a high stand, not the jack.
Start the mower, releaase the brake and look at the belt and in particular the rear drive pulley
Pulley spinning, then try changing gears and see if the wheels spin
If not then get some one to work the gears & forward/ reverse and watch to see what is moving at the control levers on the gearbox.
You might need to remove the right rear wheel for better access.
Follow the levers & rods to the bit that actually goes into the box, give it a squirt with you favourite penetrant solution and contine working it till it moves freely and fully to the stops.
