Concerning Husqvarna LR120 lawn tractor. Gears will not engae.

If the unit stopped moving at once. I suggest checking the belt. It is very unlikely both Hydro-Gear units would fail at the same time. check under the mower, and check on top of the transmissions, you should see the drive belt running between both transmissions and the engine.

If there is too much play in the drive belt due to belt stretch or linkage wear, the belt will be too loose to drive the tranny but still cause the tranny input shaft to rotate preventing it from being put into gear.
Do the gears grind when trying to put into gear? With engine off you should still be able to push the machine if you put it in neutral. Unless, of course it is a hydrostatic transmission, in which case there should be a lever to push/pull which will put the hydrostat in neutral. This is all assuming that it is a hydro and while running the machine won't move when putting the drive lever into forward or reverse.

either the drive belt from the engine to the top of the transmission has broken or gone slack due to a tensioner issue, or the transmission has failed. with the engine NOT running you can put your hand up under the deck and feel the topmost belt, if it is there and snug, then you have a transmission problem and that requires a replacement.

When you raise the front or rear of the tractor, and push on the belt (engine off of course), is there a lot of play or slack in the drive belt. At this point from what I have read, the issue is the drive belt, and is slipping when in gear and when it heats up. The next thing to do would be to replace the belt, but I not sure if this procedure is an option for you. When you inspect the under side of the tractor, and inspect the moving parts and pulleys, if any of these components are in need of replacement, you will be able to tell by either rotating them manually or visually inspecting them. At this point due to the low hours, I suspect the drive belt is stretched out, and is considered a maintenance replacement part procedure.

the belt is smoking because a pulley isnt spinning like its supposed to. its gonna break the new belt if you don't remedy the problem. try taking the belt Back off. then see if you can spin the pulleys by hand with no belt .

those fluid drive transmissions fail on quite a few brands as they all use the same ones....replace the trans...they are expensive but still cheaper than a new mower.

the belt is bad if not there is trouble in the drive shaft.Thank you for asking and I hope this helps you.Please leave me a 4 thumbs up.

If the shifting is easy when the engine is off you have an adjustment to make on the clutching mechanism. if you loosen the belt (the adjustment is under the mower) it should improve the shifting but do not loosen it too much to where the belt begins to slip when in drive.

you need to put dope in trans and check gear shifter to see if it is loode if not check your drive belt

The front belt should be a 954-0280A, rear 954-0370. When you go to higher gears, the front belt should be getting tighter as the vari-speed pulley is moving further back. Remove the shift plate, put the transmission in neutral. Start the engine, put speed selector in highest gear and slowly let out on the clutch. You should be able to see what the problem is. It will do everything the same as if were in gear, except move. Make sure you have the correct belts.