Hello there, Tankbach.
The following Knowledge Base article offers up some great troubleshooting recommendation for initial troubleshooting:
iOS: Understanding multitasking
http://support.apple.com/kb/ht4211
When performing Step 2, I would recommend closing all open apps:
If an app doesn't respond to your input, or doesn't perform as expected, do the following, testing after each step:
- Check for app updates.
- Force the app to close.
- Restart your device.
- Update your iOS device.
Additionally this article reviews some additional steps as well:
iOS: An app you installed unexpectedly quits, stops responding, or won’t open
http://support.apple.com/kb/ts1702
Thanks for reaching out to Apple Support Communities.
Cheers,
Pedro.