Force Restart iPhone or iPad without a home button
If your iPhone or iPad has a Face ID (and it doesn’t have a home button), you can force it to restart quickly by pressing the volume up and releasing it, pressing the volume down and releasing, and finally pressing the power button. After the screen turns off, you can release the power button. The Apple logo will appear on your screen and your iPhone or iPad will reboot.
The same procedure can also force restart the iPhone 8 or iPhone 8 Plus. Just press Volume Up, Volume Down, and hold the side button until these phones reboot.
Force restart an iPhone 7, iPhone 7 Plus, or an iPod touch (7th gen)
The hard restart key combinations for iPhone 7, 7 Plus and iPod touch are slightly different. If you have one of these devices, you can force a restart by holding down the side (or top) button and the volume down button until the Apple logo appears on the screen.
Force restart an iPad with a home button, an iPhone 6s or older device, or an iPod touch (6th gen)
If you have an iPad, iPhone 6s (or earlier) or iPod touch (6th generation) with a home button, you can quickly force a restart if necessary. Just press and hold the home button and the side (or top) button until the Apple logo appears on the screen.