I was replacing my driver's seat cover last weekend. Got the seat out and cover on without a huge amount of trouble....then I went to put the seat back in. I got two seat hold down bolts and the left rear hold down nut on fine, but when I went to put in the front right bolt, about half way in, the nut under the floor broke loose.

Now all the bolt does is spin. I cant get it out or tighten it and I can't get to the nut inside the floor. And the seat flexes back under acceleration....... Any ideas as to fix this situation?
Thanks for any help.