"Operation invalid without a current index"

I have a database (Access 97) that is split into front end (on several PCs on
the LAN) and back end (on the server). I need to add new fields to an
existing table. When I open the back end DB the buttons that should say Open,
Design and New are all blank. When I click on the Tables tab I get the
message "Operation invalid without a current index". What does that mean, and
how can I satisfy the requirement? If I go to File, Database Properties I get
the message "Microsoft Access is unable to load the database properties". I
have checked LDB Viewer and have verified that no one is using the database.
I can open the front end DB and see all the data and all the linked tables.
All the forms, queries and reports that I checked seem to work just fine. I'm
stumped. Please help!