Based on your warning, all categories are no longer present.
Couple things that could happen:
1. Cache got corrupted.
Solution: Try to clear cache in admin and categories will be reloaded from database.
2. Categories got deleted from your database:
Check in admin if you see categories
Solution: Restore categories from backup
3. Your database got corrupted
Solution: Contact your hosting company
Possibly something else, but it will needs to be investigated.