Categories have Vanished….

I have version 2.6.2

and all has been wonderful until today ??
Went to the site and the categories have all vanished.

If I go to admin, it says no categories, all back to uncategorized.
But if I try to re-enter the old categories as new ones, it says error, that category already exists, even though they are not showing up.

This has effected my entire blog as I have a special video category and featured links..

How do I correct this?

Martin Salter