For iOS 7.0 or later:
- Double-press the Home button.
- Swipe up on the app
For iOS 6.1.4 or earlier:
- Return to the Home screen, and then click the Home button twice.
- Tap and hold on the app.
- When it starts to jiggle, tap the to close it.
- Double-click the Home button and try opening the app again.
Note: If you are using an iPhone 3G or earlier, see this article for information on force quitting an app.