Hello,
I had a similar issue… i had to force quit FCPX and when i restarted none of my Projects were available in the Library I was working from.
In Finder i went to the library and compared the file structure to other folders and found I an extra file called ‘CurrentVersion.fcpevent-journal’ along with the usual ‘CurrentVersion.fcpevent’ file.
SO I just deleted the ‘CurrentVersion.fcpevent-journal’ file and suddenly all my projects reappeared in FCPX browser.
I hope this helps someone.