Sales Reports may not be available until you use the Refresh Statistics feature to generate the data necessary for reporting (ie: Reports may be blank if you have not run Refresh Statistics at least once recently). This feature can be accessed from the Magento admin section by navigating to Reports > Sales > Refresh Statistics.
Simply check off all the statics you need to process, then select the time period you want to refresh for (Lifetime or Last Day) from the *actions* drop down in the top-right, and click Submit.
Note: Generating Lifetime Statistics is very resource-intensive and may adversely effect the performance of your store while the process is in progress. This can take up to several minutes.