We need the complete model of your cub.
But I've got a Cub and it's common for the clutch spring to fall off when the drive belt gets too loose.
You can download a parts lists that shows the spring to see where it attaches or remove the deck, elevate the mower and you can see where it attaches to the swivel drive pulley assembly and hooks forward to hood tension on the clutch/brake drive assembly.
Hint: I installed a wire through the center of the spring on my cub and tied the wire snug to chassis so as the spring would not get LOST if it comes unhooked again. (it will stay with the machine instead of falling loose onto the top of the deck)
Here is a link to Cub Cadet models that shows 40 or so models using the search function within if you cannot find the model to yours you can choose your lt 1040 1046, etc in the search bar, or year year and try for a model, maybe.
Parts Lookup and OEM Diagrams | PartsTree
or google
Parts lookup and repair parts diagrams for outdoor equipment like Toro mowers, Cub Cadet tractors, Husqvarna chainsaws, Echo trimmers, Briggs engines, etc.
www.partstree.com