I might be able to give you some advice if you can provide a better detail such as the error description.

How do you interact with the report is it thru the Report Manager, is there a job that actually execute the report, a snapshot?

All reports run in the context of a user, I figured the user account must have been disabled or removed from your active directory. If you still have access to the report than chances are you can update the data source and change the credential being use by the report but firstly verify if the account has access to the database or server to be access by the report.