Try a "forced" restart of that iPhone, as per these instructions:--> Force restart iPhone – Apple Support.
If the issue persists, it might 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:--> Apple – Location Selector. After checking it out, they'll give you all the available options.
Regards
Giulio
(*) You can do it: