On many devices, holding down the Power button for more than 10secs will do a “Soft Reboot”.
Where it is just telling the phone to reboot.
If that does not work, the device may need a forced reboot.
For phones with a removable battery,usually, u could just pull the battery, then put it back in and power on. But most phones do not have such a design anymore.
Try holding down both Power button and Vol Down button for at least 10secs.
(for some devices, it may be Power + Vol Up)
This forces a reboot of the phone.
It will not wipe any data or delete anything.
You may get a menu screen that is the Recovery Boot options.
U would select 'Reboot System" or similar to reboot the phone.
Vol up/down moves the selection, while Power would be “Enter”.
If that fails to reboot the phone, then, unfortunately, there may be a hardware failure.