There is a view (at least in 2008) in the SSRS database called ExecutionLog or ExecutionLog2 that may help.
There are also text based trace (log) files on the Reporting server. The path to these files in 2008 is Program Files\Microsoft SQL Server\MSRS10.MSSQLSERVER\Reporting Services\LogFiles. It may be something like Program Files\Microsoft SQL Server\MSRS9.MSSQLSERVER\Reporting Services\LogFiles or some other minor variation. These log files can be very helpful, but very difficult to work your way through. Make sure you have the last time the report tried to run, and the name of the report to help you find the entries you need.