usually it is because it has either stretched, wrong size belt, or when it was put back on it didn't get routed properly through the guides that help hold in in place when it is disengaged
Disingage the mower deck take off the mower deck look under neath their and replace the belt
You may have installed the wrong size belt? One of your spindles may be bent. Double check to see that your spindles are not bent and that you have purchased the correct belt for your mower.
You have probably broken the drive belt. With the mower turned off, lie on the ground and look underneath the mower. There is a deck belt and a drive belt. The drive belt is the upper belt. Grab the upper belt and see if it is broken.
here is a diagram that might be helpful: https://jdparts.deere.com/partsmkt/document/english/featbene/L120_L130_48_3N1deck.pdf
You can change the drive belt, on your lawnmower, by loosening the tensioner pulley. With the tensioner pulley, loosen the drive belt will come off and can be replaced.
I have Murray 42" cut riding mower that the belt came off. How do I put the belt back on?
There are two things that could cause this. The first and most popular problem is that the safety switch for the seat is not working correctly. It determines whether someone is on the seat. If for some reason the driver should fall off of the mower while mowing it will shut the mower off. It is a good feature but it causes the problem you are talking about when it fails. The other thing it could be is a problem in the deck. One of the spindle could be seizing and keeping the deck belt from turning. That would bog down the engine and make it stall. This is rarely the problem. You can rule this out by taking off the deck belt and starting the mower, lowering the deck and see if it still shuts off. If it doesn't then this is the problem.
You will need to remove the cutting deck to get to the blades. Here you will find the drive belt, which you just slip off.
As your sitting on your mower look down on your left side on the deck is a metal arm locked under a hook get off the mower push down on the arm and pull out toward the rear tire this loosens the rear pulley now put belt over the top front pulley and over the rear pulley move metal arm back to hook and lock in place then your done.
When working on a lawn mower, always make sure it is turned off, the spark plug is disconnected and the positive battery terminal is unhooked for your safety. First remover the belt keepers from around the belt. Next, you will be able to remove the belt from around the front pulley. Remover the deck from the mower by removing all the pins that keep it attached. You will now be able to remove the outer deck covers and loosen the remaining pulleys. Now you will be able to remove the belt and put the new belt on. You are ready to put the mower back together.
You'll need the snowblower, the mule drive for the snowblower, and the belt off of your mower deck. The blower attaches where the mower deck mule drive snaps in with the two spring pins. The mule attaches to the bottom of the blower and back to the deck height adjustment hookup. Run the belt from your PTO, through your idler pulleys on the mule, then round the drive puller on the blower. Tension the belt and you're off. Case tractors are beautifully simple.
Remove the mower deck by the quick disconnect latched and the front bracket, and the lift linkage ARM and take the belt off the engine pulley and leave with the mower deck. Remove the mower deck plastic guard to access the timing belt. Loosen the bolts on the right side tensioner bracket and back off the tensioner adjustment bolt all the way. The timing belt can now be removed. The center forward guide pulley can be removed to make it easier to reinstall the new belt. O had to remove the brake and the top pulley from the left deck blade pulley to get the new timing belt on. Put the new belt on with the blades 90 deg from each other. I set my tensioner tight now that I have the lawn defined and no more pioneering. If you hit an object the timing may slip and the tensioner has to loosened to get the 90 deg back.