Restart your device
Restarting your iPhone/iPod/iPad can often cure a lot of potential problems.
Reinstall the app
It may be that the app's data somehow got corrupted and is causing the app to crash. If this is the case, you should try uninstalling the app and downloading it again. Rest assured you will not be charged again; once you buy an app you can re-download it as many times as you like. To uninstall an app, you need to hold down its icon until it starts jiggling and then press the little X.