My iPhone X won't turn on so I can't back it up

What WheelieNick said- here are some specific instructions. With iPhone X, press and very quickly release the volume up button, press and very quickly release the volume down button, then press and hold the side button for about twenty seconds. When the Apple logo appears, release the button.

The timing is a little tricky, so try it a few times. If you can't force restart it, it's a hardware issue.

Force restart iPhone - Apple Support

Try to charge the iPhone and force restart it. If it will then not turn on after that, you cannot backup right now.