FCPx replaced media with Gaps

Can I throw a spanner in the works and suggest you haven't accidentally converted your clips to gaps. In your screenshot all the clips in storylines have converted to gaps, but anything connected to a storyline has remained as it should.

I can't think how you could have accidentally selected only everything in storylines before accidentally hitting shift-delete.

A while back, not on the latest version of FCPX, I had about 5 or 6 clips in a longer project convert to gaps for no reason, but it was a small percentage of what was there. I had a backup system in place and just reverted to a previous project version. It could be a bug.