Q: 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!