that happened to me a while back and it turned out to a peice piece of dodgy geometry...

go through and hide each piece one by one and see if will render.
with me it would even start a render, would abort before the GI was calculated, so when it started i knew what it was.
its a little bit annoying but better than re-doing the whole thing

on the other hand it might be something totally different!
good luck!

pat