Hi Sahana,
What is the datasource of the report?
What version of CR are you using?
If the 'Optional Prompt' option is set to 'False', there is no way the report can run without entering/choosing a value in the prompt window.
Do you see this behavior in the CR Designer or the BO Enterprise? When you save the report to the Enterprise, make sure that the option 'Save Data with report' is unchecked (File > Save data with report).
-Abhilash