Hi, LEGOers,
You didn't mention what version of iMovie that you are using. Some older versions do not work well with El Capitan.
The current iMovie verion, 10.1.2, is recommended.
Trouble shooting can be time consuming. Below are some things that you can check or try. For safety, I keep a backup iMovie library on an external disc.
Delete preferences by restarting your computer, and launching iMovie while holding down the option and command keys. Then select the delete preferences option in the resulting dialogue box.
2. Launch iMovie in a different user account. You can use your Guest account for this.
3. Make sure that you have sufficient disk space for iMovie to operate (25 GB frequently is recommended.)
4. Check to see if some third party app (perhaps recently added) might be causing a conflict.
5. Restore iMovie from a Time Machine backup to a point in time when it was working properly.
6. Reinstall iMovie from the App store.
Good luck with this. You might find that it is something simple that is causing the problem. Or, if you are having difficulties with other programs as well, it might be a hardware problem, in which case your Mac has some built in
diagnostics that you can use to identify the issue.
Best,
Rich