Hi,
Can you explain in more detail what you mean when you say that "iMovie won't work"? Is it not opening? Or is it opening and won't function? Are your projects gone? Are you getting any error message and, if so, what is the exact text of it? Do you have sufficient space on your internal drive for iMovie to funtion? Click on Apple Logo/About This Mac/Storage and see how much free space you have. You would want at least 10GB of free space.
Try opening iMovie in a new library by holding down the Option key as you open iMovie.
Try also booting up in Safe Mode and, after everything loads, shut down and reboot in normal mode. See if that cures it. Here's a link with instructions:
Start up your Mac in safe mode - Apple Support
-- Rich