If a report returns blank results, check to ensure that you do not have a blank field in Tools > Edit Settings > Drop-down Lists. Additionally, it is very important that you do not delete or edit a Drop-down Field. Check to be sure that the search criteria has not been edited or changed in some way.
Example: A Session that has been changed from 2011-12 to 2011-2012 in Tools > Edit Settings > Drop-down Lists will create an issue for Jackrabbit when searching.