Hi,
Assuming that you are using iMovie 10, the first thing to try is to open iMovie while holding down the Option and Command keys, and selecting to delete preferences in the box that appears. Sometimes that cures glitches.
It's also possible that there is some corrupt frame or clip in the movie that is causing the share to fail. In that case you would need to scroll through the movie looking for white flashes, black frames or artifacts indicating corruption, and cut those out of the movie.
You can try copying your media into a newly created project and see if it will share from there.
Make sure that you have sufficient space in your destination drive to contain the movie, as insufficient space can cause a share to fail.
Another thing is to create a new library, copy your project into it, and try to share from there.
-- Rich