Starting with the iPhone 7 with the new home button using the old keystrokes you might have used to reboot your phone will no longer work. On the other hand you definitely can reboot your phone it just takes a little bit of extra knowledge to do it.
On the iPhone 7 you have to know the magic steps:
1. Press and hold the on off button and while you're holding it
2. Press the volume down button and continue holding both buttons until your phone reboot
Wth the iPhone 8 it's different. I don't know why on the iPhone 8 you:
1. Press the volume up then the volume down
2. Then press and hold the on off button until it reboots.
Good luck remembering all bad in any case you can always search "reboot iPhone 8 or reboot iPhone 7" online and you'll find those answers.
To reiterate for the iPhone 8 press volume up volume down and then press and hold the on off button until it reboot. Easy to memorize but obviously impossible to figure out on your own.