Captive nut access, front subframe driver side...

I have a failed subframe mount on the driver side, front mount of my 1995 Sable. THe subframe is hanging an inch or so low to that side and a massive clunk is apparent when I accelerate from a stop. I was removing the bolt that goes thru the mount, subframe and into the body rail above. I got a few turns successfully, then the captive nut in the hollow body rail started to spin, now I can't remove the bolt! What is the method to access the nut so as to put a wrench on it? This is the area below the battery, which I will be exploring soon!!! Any tips?