iOS 12
Quit and restart an app on iPhone
If an app isn’t working properly, you can quit it and then try to reopen it. Restarting the app may resolve the problem. (Typically, there is no reason to quit an app; quitting it doesn’t save battery power, for example.)
To quit the app, open the App Switcher, swipe right to find the app, then swipe up on the app.
To restart the app, go to the Home screen, then tap the app.
If restarting the app doesn’t solve your problem, try restarting iPhone.