My iPhone X doesn't turn on

Hi there,

Please try this

1. Press and quickly release the volume up button.
2. Press and quickly release the volume down button.
3. Press and hold the side button until you see the Apple logo (this takes around 10 seconds).
4. If your iPhone doesn't turn on, charge your iPhone for one hour and then try again. If your iPhone shows a low-charge battery, check your hardware and charge your iPhone again.
5. If your iPhone still doesn't turn on, contact Apple Support.

See this link for further reference

If your iPhone won't turn on or is frozen - Apple Support

try to press the button

When was the last time that the battery was replaced on your iPhone?