Hi Sean,
the same happened once to me. The project may get corrupted if it was in the process of making a back-up. Where do you save your back-ups of your projects? It should be a folder called “Final Cut Backups” (I store it on my internal drive under Movies).
Click on the library ->Click File-> Open from Back up and choose your last back up. You will have to relink your media (I had to).
After restoring your Event/project from the back-up you might want to duplicate it and continue your work with the duplicate just to be on the safe side.
Anna