Updating iMovie should not cause links to be broken. When you updated iMovie it may have opened in a new blank library. So, open your Movies folder in the finder and confirm that you are in the correct library. If not, close iMovie and click on the correct library to open iMovie. Your edited projects should be there.
If still no luck, try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. As described above, reopen your old library to get back to your projects. Now see if your links are established.
Failing that, try shutting down your computer and booting up in Safe Mode (see Finder help menu for instructions) and, after everything loads, immediately shut down and reboot in normal mode. See if that cures it.
You also can try redownloading iMovie, as that often cures glitches. To do that, drag your iMovie app from the Applications folder to the Trash, but don't empty the trash. Then sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You should see displayed all of your purchases, including iMovie. Redownload it from there. If for some reason the redownload is not available or is unsuccessful drag your old iMovie from the trash back into the Applications folder.
One final thing you can do if nothing above worked, is to try to recover your projects from the iMovie Backups folder on your Mac. To get there, follow this file path from the Finder menu:
Go/Home/Library/Containers/com.apple.iMovieApp/Data/Library/Caches/iMovieBackups
When you get there you will see a list of previous backups of your iMovie library. Click on one that predates your issue and iMovie will open in that library. Navigate to your projects and see if they are playing properly.
Everything else failing, try calling Apple Support. Click on the Contact Support item at the upper right hand corner of this forum's screen, and navigate to a telephone number. The tech persons there are pretty savvy and may be able to solve your issue. Also, they have the capability to remotely access your computer, that we cannot do here on a consumer forum.
Good luck with this.
-- Rich
