imovie freezes uploading movie to icloud

For several days, iMovie has been frozen during a home movie import. I selected the movie from my library and attempted to share it using iCloud. I was able to upload several other movies to iCloud without any problems, but this one is stuck. The entire iMovie app is frozen; I cannot quit the app, and I cannot select any menu items. The iMac works fine in all other apps I tried, but not in iMovie. I may come back to you for help on the upload part once I get the app unfrozen. How do I unfreeze the app? Thanks!