Before following steps in this article, make sure that you have current backups of your libraries and your media, as well as a backup of the iMovie application.
These steps might not cover your specific issue and are not comprehensive.
1. Restart your computer
Restarting your computer resets iMovie related system resources. It's the simplest thing you can do to resolve iMovie issues.
2. Ensure iMovie is up to date
If you're actively working on a project, avoid updating or upgrading iMovie until you are finished with the project. Either complete the project in the version of iMovie you are currently working in, or make sure that you have a current backup of your events, projects, and iMovie. Having a back up ensures that your data and the application are unaffected if updating or upgrading to the latest version of iMovie doesn't address your issue.
3. Review iMovie documentation
To verify that iMovie is working correctly, it's always a good idea to review the documentation to ensure that you clearly understand the expected behavior of what you're trying to do in iMovie. Following this step thoroughly could save a lot of time by eliminating perceived issues.
For information on how to access iMovie Help see iMovie help.
4. Make sure your camera is supported for use with iMovie
Make sure your camera is a supported camera.
5. Isolate an issue by using another user account
For more information see how to test an issue in another user account on your Mac.
6. Create a new library
You can isolate issues with libraries by creating a new, empty library when opening iMovie. This demonstrates that iMovie is working as expected with a new library. Then, you can open and close your libraries one at a time until you can to reproduce the issue and identify an affected library.
To create a new library, first quit iMovie. Then, option-click the iMovie application icon in the Dock and click New to create a new library.
7. Test with "known-good" media
"Known-good" media is media that you know consistently works without issues. This might be a media file from a previous project that you confidently know works as expected.
Using known-good media, such as the h.264 or MPEG-4 files from QuickTime Sample Files, can help you isolate an issue down to the media you're attempting to work with. If known-good media works as expected in iMovie but other media you've imported into iMovie does not, there might be an issue with the imported media. You might need to acquire it again from the original source, or it might be media that isn't supported for use in iMovie. Learn more about supported cameras.
8. Remove and reinstall iMovie
For instructions on removing and reinstalling iMovie, learn more about installing, updating, and uninstalling apps on OS X El Capitan.