A. Force close the app by swiping up from the bottom of the screen (or double-clicking the home button) and swiping the app off the screen. Then, reopen the app. If it still won't open, restart your iPhone by holding down the power button and sliding to power off. After the device restarts, try opening the app again.

B. Check for any available updates for the app in the App Store. If an update is available, install it. If the app still won't open, consider uninstalling and reinstalling it from the App Store.