If your iPhone isn't responding, and you're not able to power it off and then on again, you can force restart it to hopefully fix the issue.  Forcing a restart is different from a normal restart.

If your iPhone shows a black, unresponsive screen, start by checking for physical damage or charging the device for at least 15 minutes. If that doesn’t help, force restart your iPhone or update its ...

If your iPhone won’t turn on, it can feel like the device is completely dead. Sometimes the screen stays black, sometimes the ...