Try to force quit the app.
Double tap the home button. When you see the app previews on the screen scroll sideways if needed to get the preview in question on the screen. Flick that preview up and off the screen to close the app. Single tap the home button or home screen preview to exit the mode.
Did you recently update the app? If so you may want to look at the app's page on iTunes. It's possible that the update could have broken the app. If you look at the reviews - change the sort to most recent instead of most helpful - then you may see that others could be having issues.