No users showing in User Manager

In NT4, whenever we open User Manager, none of our users are displayed. Groups are displayed at the bottom but no users at the top. If you try to add an existing user you get an error message that they already exist–but they are still not displayed. How can we bring back our list of users?
I’ve tried restoring with the ERD but they still are not displayed. Installed SP6a and it didn’t correct it either.
Help.