A large spring fell off John Deere LX277 lawn tractor and tractor won't move Where does this spring belong and how is it attached?

This is probably the large spring that tensions the idler pulley assembly for the main drive belt (the belt that runs whenever the engine is operating and runs between the upper engine pulley and the pulley on the transmission. It hooks on the idler pully assembly on the left side of the mower and then on a vertical bracket toward the front of the mower. You'll need a big screwdriver to get it back on. Put a socket wrench on one of the idler pulleys and pivot the entire idler assembly to release tension on the belt so you can snap the spring back in place with the screwdriver.

Uncle Al