Redownloading iMovie would be one thing to try. Sometimes that cures issues. Before doing it there are a few other things you can try.
You could try booting up in Safe Mode (see Finder menu for instructions) and, after everything loads, immediately shut down and reboot in normal mode. Then see if iMovie works properly.
Check to make sure that you have sufficient app free space on your drive for iMovie to function. Sometimes space can be used up without us being aware of it. Insufficient space can cause iMovie to crash.
Also, try the following procedures, as they are very easy to do and sometimes can cure crashing problems. It simply a matter of shutting down and rebooting while holding down certain keys.
Resetting the System Management Controller
Try resetting the System Management Controller (but no longer available for the new M1 computers):
https://support.apple.com/en-us/HT201295
Resetting the NVRAM or PRAM
Try resetting the NVRAM or PRAM:
https://support.apple.com/en-us/HT204063
-- Rich