help needed with delphi and databases

Could any one please tell me why I might be getting the following error in a
project I am trying to write....

raised exception class EAccessViolation at address 004026B6 in module
'project.exe'. Read address FFFFFFFF.

This error is produced after i run the project and reach a page that uses a
query to fetch details from a database. On the first occasion of usin gthe
query it fetches the correct entry and moves on to next screen but if I try
to go back to the first screen to use the query again, then the above error

I will be extremely grateful for any help

Thanks Phil Morris