If catalog broke and can't be fixed, you have to replace it with latest backup.
But consider saving in case somebody can fix for you, e.g. Adobe, e.g. Paul K..
If no backup, then rebuild catalog from scratch (hopefully augmented from xmp).
(if no xmp - you may be screwed, but there may still be hope - do tell..).
In case not obvious at this point, it is prudent to:
* backup catalog regularly (everybody should do this).
* save xmp as you work (some people don't do this, but you're living close to the edge if you don't).
Got backup to open Still have to redo a lot of work but at least most of it is there.
Got backup to open
Good to hear!
Still have to redo a lot of work.
If you save (xmp) metadata as you go, then you never have to redo editing work when falling to backup catalog (just sayin'..).
PS - all develop settings and almost all metadata will be in xmp if saved, but not everything from catalog is contained in xmp (and not everything in Lightroom is contained in your catalog!).