If you do experience problems using an app or a game that you purchased from Marketplace either on your Window Phone 7 or through the Zune software on your computer, then this article may interest you.
To troubleshoot any of the issues you may encounter, you’ll have to try the steps in these six possible solutions;
1) Verify the application requirements
2) Restart the phone
3) Check for application updates
4) Remove the app or game
5) Reinstall the application
6) Contact the app publisher
After each one, check to see whether the problem still exists before trying the next steps.
For detailed steps on the the six possible solutions, visit Microsoft KB2508228.