It might be broke. Try holding the power button AND the home button, If that doesnt work, and you still seek answers, go to your nearest apple store, ask for help, and they will start to examine it. After a while, they will tell you if its dead or not. Best of luck!
You can force restart your iPhone by pressing and holding the Home and Power buttons. If no use, use iTunes to restore your iPhone. Or use TunesKit iOS System Recovery to fix the issue without erasing any data.
