iPhones do not crash often, but even Apple products are not safe from fails of modern technology (insert anti-fans’ moan). Older iPhones’ way of restarting was to press’n’hold Hold and Home buttons until an Apple logo pops on-screen. Instruction on how to do that is a bit different in iPhones 8 and 8 Plus, though:

– press and release Volume Up,
– press and release Volume Down,
– press and hold Hold/Sleep/Power until the Apple logo appears on-screen.

If my simple and clean instruction is still somehow unclear to you, watch this video.