Occasionally a buggy app or a new iOS update will result in instability that sometimes causes your iPhone to freeze or become otherwise unresponsive. Instances of this sort require you to force a reboot (aka hard reset, hard reboot, force restart, force reboot). A force reboot will retain your data, so it's not like doing a factory reset which erases everything. Hopefully after restarting you will be able to get rid of the offending app or revert to an earlier more stable version of iOS.
Older iPhones required the Home button to be held down along with the Sleep/Wake button to force a restart, but the iPhone X and later has no physical Home button, so there is a new method (that is also used by the iPhone 8 and iPhone SE). Here's how to force reboot your iPhone:
- First, quickly tap the Volume Up button.
- Now tap the Volume Down button.
- Press and hold the Side Button until you see the familiar Apple logo that means it is restarting.
You have to do the first two steps quickly, so keep trying if it doesn't work at first.