[Patrick Mcintyre]

I didn’t think this of myself but got it from a post by Tom Wolsky ( thank you Tom even though I do not know you ):

1. close all of your FCP libraries.
2. create a new empty library.
3. Re-import the ‘missing’ files. in my case they were imported from an iPhone. Make sure they are completely imported and transcoded by watching the background task window
4. Eject the iPhone/camera
5. Re-open the problem libraries and delete the missing-camera/problem clips
6. Drag the clips from the newly created library to the old library. They should get replaced in context without re-creating and re-editing each project

You may want to back up you library just in case before you try this.
No promises. I hope this works for you. I was very frustrated and not looking forward to re-creating a whole project. This worked like a charm for me.