mps9506

This probably sounds stupid as heck, but here I go with it...
My buddy was helping me swap my stock seats back into my civic. He had trouble tightening down one of the bolts that holds the passenger seat in, the one nearest the door towards the rear. He was cranking down and the nut on the other side of the floorboard broke off, as far as I can tell. I cannot figure out how I should get to this location though to weld the nut back on. there is some kind of plastic or another peice of metal that covers up everything under the floorboard it seems. I don't want to cut it becuase I don't even know what it is yet. Wanna get this fixed because I don't want my passengers sitting in a seat with only three bolts holding it down.
Thanks...