Sounds like the nut came off of the spring tie bolt, either way this bolt does not hold the spring to the axle or U bolt plate, it does have a stud on one side to align the axle in the right spot on the spring (on top for axle above spring or bottom axle below spring). I would go to a auto parts house with said bolt and get a new one to replace it with, For this one needs a couple of C clamps to pull the springs back together with bolt in place minus the nut, once springs are back together the nut can be replaced.
Hope this helps some.
