iMovie will not export

I have been attempting to export a 1.5hr movie to a 720P format unsuccessfully. It will basically "Create the HD 720P" for 5-6hrs and then it will repeat the cycle but much faster then quit all of a sudden.

Long story short, it doesn't export anything.

I have a Mid-2011 Macbook Air. I have tried exporting to the internal and to an external hard drive. Nothing is working thus far. I have successfully exported a movie of this length before. The only thing that is different this time is that I only have 14GB of free space on my internal drive. Could that be it?

I would appreciate any help. Thanks!