The Lost Leads & Opportunities Summary report can be found in the Reports module in the Sales Reports section.
The report enables you to see the reasons you lost leads / opportunities, the total count of each, and how much percentage of lost that reason accounted for.
This report has many filters available. The results will show where all the filters are met, and summarizes them for you.
What date does the date filter use?
The results are summarized based on the date the lead / opportunity was actually marked as lost.