Thought I should advertise Apple's page:
iOS: Troubleshooting applications purchased from the App Store
If you are having an issue with an app you purchased from the App Store on your iPhone, iPad, or iPod touch, try the following troubleshooting steps to resolve the issue.
Try the following steps to resolve your issue, testing the app after each step.
Update your device software
Install another app from the App Store
Restart your device
Check for app updates
Reinstall the affected application
Report an issue