does the report use the parameter to filter the data? what sort of values? does Crystal 'know' how to interpret what the user entered? does the report have static text so that if no data is returned, a report will still be displayed?
There isn't alot to go on, given what you described. What exactly is 'nothing happens'? You get a blank page? does it have the header and footer from your design? I am assuming that you dropped fields on the report, does that header show (Crystal auto generates a column header when you drop the field, or has this been modified?) Is there any suppression in place?