iPhone won't restart OR force-restart.

Try this steps:

1. Connect your iPhone to a computer. On a Mac with macOS Catalina, open the Finder. On a Mac with macOS Mojave or earlier, or on a PC, open iTunes. Find out which macOS your Mac is using.
2. Select your iPhone on your computer.
3. While your iPhone is connected, follow these steps:

• On your iPhone 8 or later, including iPhone SE (2nd and 3rd generation): Press and quickly release the volume up button. Press and quickly release the volume down button. Then press and hold the side button.
• On your iPhone 7 or iPhone 7 Plus: Press and hold the side button and volume down button at the same time.
  • On your iPhone 6s or earlier, including iPhone SE (1st generation): Press and hold both the Home button and the side button or the top button at the same time.

1. Keep holding the buttons until you see the recovery mode screen.
2. When you get the option to restore or update, choose Update.
3. Your computer will download the software for your device. If it takes more than 15 minutes, your device will exit recovery mode. Repeat steps 2-4.

If you need more help

Contact Apple Support to set up service.

If you already tried (to no avail) a forced restart of your device using these instructions:--> Force restart iPhone – Apple Support, 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:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.