Cratsman lt2000 lawn tractor will start but will not move forward or backward. I tried changing the drive belt, but that didn't work.

You did not install the belt correctly, try again!

I would lower the deck to the lowes setting and look between the frame and the deck toward rear wheels and check the drive belt it sounds like it is slipping or busted.

Your drive belt is off or slipping.

Have you Inspected your Belt? Your Drive Belt and Pulley's need to be inspected

Check the drive belts first. and make sure the hydro release lever is in the correct position. If it is not- you can push the lawn tractor and have the rear wheels turn. It sometimes doubles as a "Forward" "Neutral" "Reverse" lever on other brands of lawn tractor.

The belts should be in their guides and pulley grooves- check to make sure it does not have a hidden break by having the engine off, and pulling on the belt.

Lastly, check to make sure the hydro itself is not out of oil or low on oil. It uses low viscosity "HyGard" oil. Capacity if you drain the old oil out and refill is 4.6 quarts. Look for a leak if the level is low.

If it still does not move, make sure the levers are set so you are in forward motion positions, and push- if the rear wheels move there is a problem in the hydro unit.

when you took off the bag did you pull the lever that allows you to push it around the one that puts the hydro drive in neutral its on the back a couple inches away from the eyelet you can tow with it should be released forward

Check shear pins in the blades . Tyr turning by hand with power shut off. If you can turn by hand then I am sure the shear pins are broken. Check drive belts as well.

1) The drive belt between the engine and transmission has broken or jumped off a pulley.
OR
2) Your hydrostatic transmission is dead.

DO NOT STICK YOUR FINGERS UP INSIDE A RUNNING MOWER TO SEE IF THE DRIVE BELT IS MOVING.

check drive belt and tension pulleys under machine if belt is stretched or damaged replace if popped off pulley check all to make sure one has not froze causing it to pop off also check rear pulley on axle sometimes pops off and just spins

the branch probably got cought in the belt that runs the rearend it may have caused it to come off check the drive belt