I had a similar occurrence with one of my shocks when the lower nut loosened and fell off. The lower mounting bolt came out of the mounting plate and just hung there. There was no apparent damage to the shock itself and we drove several hundred miles with only three without problem. If fact I could not tell the difference.
You can purchase a mounting kit, Monroe part no. P1134, which includes the upper and lower bushings and the nut, for about $1.75. In this area O' Reilly Autoparts is a Monroe distributor but they had to order them and shipping is not include in the price. I purchased two sets.
Replacement is straight forward. The shock compresses quite easily, In my case neither of the brackets were damaged, and I used loctite on the thread when replacing the nut.
Hope this helps,
David E