Reset the iPhone
Generally, the iPhone is a very stable device and most people will not have any issues with it. However, if you never turn off your iPhone or you habitually download new apps to try, you may find yourself in a situation where your iPhone seems to be running slow. It may even lock up or freeze while you are using an app.
In many cases, simply restarting the iPhone by powering it off and on may clear things up. The iPhone is powered off by holding the Sleep/Wake button on the top right of the device for a few seconds until the power off slider appears. If you have an iPhone 6, iPhone 6 Plus or newer iPhone, the Sleep/Wake button is on the right side of the device.
A more effective solution may be to reset the iPhone.
Resetting the iPhone can be accomplished by pressing and holding the Home button at the bottom center of the screen and the Sleep/Wake button at the same time. Beginning with the iPhone 7 and iPhone 7 Plus, you need to press and hold the Sleep/Wake button and the Volume Down button to reset the device.
Press and hold both buttons simultaneously for approximately ten seconds. While you are holding the buttons, the screen go black. Continue holding until the white Apple logo appears. You can release both buttons once you see the Apple logo. The iPhone will restart and should function normally.
If you attempt to perform a reset and the power off slider appears while you are holding the buttons, you did not manage to hold the buttons correctly. To reset the iPhone, continue holding the home button and sleep/wake button even after the power off slider appears. If you continue to hold the buttons, the screen should go blank in a few seconds. Remember to continue holding the buttons until the Apple logo appears.
An iPhone reset will not normally remove or delete any apps,data, or settings.
Force Quit an App
Occasionally, you may just be having problems with one app. You could certainly reset the iPhone to clear the issue, but it might be quicker to force quit the app. To force quit an unresponsive app, double-click the Home button to enter multitasking mode. Find the app, then tap and hold on the screen image of the app and swipe upward. The image will slide up and off the top of the screen. This force-quits the app. Tap the Home button to exit multitasking mode.
If it works correctly, your iPhone will close the unresponsive app and return you to the home screen.
To learn more about multitasking on the iPhone, please see the article “Understand Multitasking on the iPhone”.