Ideally, if you can avoid re-importing, it will be better (and in best case scenarios, you will/should be able to).
Here is one way to "re-build" a catalog without re-importing:
* select all photos (e.g. via "All Photographs collection - make sure lib-filters are disabled, and stack underlings are selected too..).
* click File menu -> Export As Catalog.
Note: I assume you have already tried optimizing and catalog integrity check.
If all attempts at salvaging existing catalog fail, and you do decide to rebuild from scratch, then make sure you save xmp metadata before embarking, that way, when you re-import, you'll not lose develop settings and other critical metadata.
Somebody else may have better/more info for you, so don't rely on this post alone..
PS - if you don't know how to do some stuff, ask how..
*** NOTE: re-building drive with photos will NOT invalidate your catalog.
Good luck,
Rob