Depending on your specific requirements, you may be able to build the report you are looking for using the built in Reporting Wizard. You can find this by creating a new report from the Workplace -> Reports area. You would likely need to have Contracts as the primary record type in the report and Cases as the related record type.
The wizard as you would expect does have its limitations, so if you are looking for something more, you can create reports using SSRS/BIDS and upload them to CRM. These reports can be build with FetchXML or T-SQL (On-Premise deployment only).
I will also like you to refer to some links below:
blogs.msdn.com/.../custom-report-capabilities-in-microsoft-dynamics-crm-2011.aspx
msdn.microsoft.com/.../gg328097.aspx
For CRM 4.0: