If your existing query has any criteria, keep that query and build a crosstab
upon it.

In the Database Window go to Queries, then New. The Crosstab Query Wizard
should be one of the options.  Click on the Both option button so that you
can see both your tables and queries. Find the original query and let the
wizard guide you through the process. Don't be discouraged if you have to try
it 2 or 3 times to get it right.

Also if you are using a parameter in the original query to prompt for
information, make sure that while in design view you go to Query, Parameters
and specifically define the parameter datatype. Crosstabs are picky if you
don't.