Invalid Index Descriptor

I am a novice with Delphi and Databases.. please help me..

I am accessing 3 files..
Delivery.dbf    108mb
Delserv.dbf       66mb
Invoice.dbf        42mb

I can access all of the files except for the delivery.dbf.

The error that i am getting is
Invalid Index Descriptor
BDE Error: 10023
[$27] [$27]

It is probably a simple problem...
 Thank you..