This MAY work. In your Final Cut Projects folder, find the CurrentVersion.fcpproject file for that timeline and trash it. In the Backups folder for that project, copy and past the time-stamped .fcpproject file up one level (same place you found the CurrentVersion file). Then rename that copy to CurrentVersion.fcpproject. Launch X. See if this back up appears correctly.
I was absolutely unable open FCPX in Project Library when broken project was in FCP Projects folder. FCPX was rebuilding thumbnail views and it crashed it always.
Backup project was not working too.
Once it opened Project Library, so I duplicated broken project three times with different options, but it didn't help. Duplicates were broken too.
Then I moved broken project away from FCP Projects folder and made fake empty project with same name and opened it. Then I closed FCPX and replaced fake project with broken one.
FCPX opened and after few seconds was crashing again, but randomly. At last it stayed opened, so I exported timeline and I'm happy now.
It was really nightmare :-) Now I have to set TimeMachine and be very careful in the future.