1. Try restarting the phone. Sometimes that is all it needs.
2. If that doesn't work, then go into "Settings" then "Apps". Scroll over to "ALL" and locate that app in the list.
3. Tap that app and then tap "Clear Cache".
4. Again restart the phone.
5. Should it still be a problem, follow step 3 except tap "Clear Data" instead.
6. Restart the phone.
Should none of this work, then you may need to consider uninstalling and reinstalling that particular app, or even doing a "Hard Reset" of the device.