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Press Command, Option and Esc keys and this will bring Force Quit. Select iMovie and force quit it. Now you can delete the video projects from Finder and start afresh.
In Finder, click on Go menu at the top bar and select Home. This will open Home folder. In Home folder, open Movies folder and you will see your videos and movie projects (Movie Library) there.
You can trash it from here. Remember, deleting this will delete all contents of iMovie. So I'd recommend to backup this iMovie Library or your movies to an external hard drive before deleting them from your Mac (if you need them). Make sure you DO NOT DELETE your important videos.
Then open iMovie and it will load a blank project, so you can edit new ones.
Please let me know if that didn't work, so I will continue to help you