If you already tried (to no avail) a "forced" restart process: put it on charge for at least half an hour, then, keeping it plugged in, use these correct gestures:
Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the side (power) button. Only when the Apple logo appears (it might take 10-20 seconds), release that button (ignore all other screens that might appear, the one with the "slider" to turn the device off included).
If the issue persists, it could be a hardware (battery? screen? other internal component?) failure but we, just users like you, cannot diagnose that issue remotely, sorry.
I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> https://locate.apple.com/choose-country-region . After checking it out, they'll give you all the available options.
Regards
Giulio
(*) You can do it: