A hard restart can be handy and should be used if an app refuses to start or if your phone has frozen up completely. The reset technique is simple, but it differs slightly depending on which model of iPhone you have.
If you have a new-style device with no Home button, you’ll need to quickly press the Volume Up and Volume Down buttons in succession. Then, press and hold the side button until the Apple logo appears.
With an iPhone 7 or 7 Plus, hold the Volume Down and On/Off buttons together for around ten seconds, ignoring the slide to power off message that appears. The screen then goes black and the Apple logo appears.
Let go of the buttons and wait for the iPhone to start up and everything should be back to normal.
If you have an older iPhone, the process is the same but you’ll need to hold the Home and On/Off buttons together instead. See the illustration below for the location of those buttons on various sized devices.