Restarting Final Cut releases and refreshes memory so that if something was confusing your system, restarting Final Cut can fix it.
2Check your media
Frequently, problems are caused by still images that are too big. Limit your digital photos to no more than 5,000 pixels on a side. Final Cut is a video editor, not a still image editor. Images that are too large cause problems.
Also, stock footage files are known to cause problems. Be sure to convert all stock footage to ProRes 422 before importing into Final Cut Pro X.
3Delete render files and re-render
Open your Project in the Timeline and choose File > Delete Project Render Files. Render files can become corrupt. Trashing them forces Final Cut to create new render files which often will fix the problem.
4Create a new library
Create a new library and a new project. Then, copy and paste sections of the old project into the new project. Test each section until you find the spot that doesn't work. At which point, you can remove or replace the bad media.
5 Create a new user account
Open System Preferences > Accounts and create a new User Account. This resets all preferences for both Final Cut and the operating system back to factory defaults. The only problem with doing this is that a new user account can't access files stored in the Home directory of the old user account. For this reason, I always recommend storing all libraries on external hard drives, not the Home Directory.
6Trash FCP X Preferences
Trash Final Cut Pro X preference files. There are at least three preference files and, if you've had a crash, there can be more. Here are the steps:
Go to the Finder.
Hold down on the Option key and click on the Go menu. The Library folder inside your Home Directory now shows up as a selection inside this menu. (You could also type Shift+Command+G, or select Go > Go To Folder in the Finder; then, type ~/Library/ and click Go.)
Open to the Library > Preferences folder.
Trash any other files that start with "com.apple.finalcut".
Empty the Trash.
Restart Final Cut Pro X.
7Remove and re-install Final Cut
If all else fails, delete FCP X from the Applications folder and re-install it from the Mac App Store.
One of these techniques should fix your problem. If not, it's time to contact Apple Support.