Sometimes it's necessary to force close an app and re-open it. Here are the steps for force closing apps in iOS and Android:
Double click the Home button (or 3D Touch press the left side of the screen on iPhone 6s) to bring up the fast app switcher.
Swipe to the app you want to force quit.
Touch the app card and hold it then flick it up and off the screen
Open the Settings app.
Touch the Running tab to view only active or running apps.
Choose the app that's causing you distress.
Touch the Stop or Force Stop button.