Not really sure with the limited information provided.. but a stab in the dark :)
Make sure to run the Crosstab Query.. make sure it works correctly. I would create a new report from the "Report Wizard" and see if you can change that in Design to get what you want.
Or.. try checking to make sure an Expression you have on the report is allowed. The field with an expression may not be interacting with the query making the whole report not run.
Or.. make sure the "Record Source" underForm properties is set right, I have in the past put that information in the wrong field within properties and couldnt find out why b/c I didnt check the data source.
Send me an email or comment if you need more help.