Have you tried restarting the phone? Sometimes that is all it needs.
If that doesn't work, I would make sure all your other apps are closed first and then try opening it. As another app could be interfering.
Another thing to try, other than the above, is to go into the "Settings" then "Apps", "All" apps and locate the app for that. Tap it and then tap to clear its cache (not data). Then restart the device.