Unresponsive iPhone X

Try and Force Restart your iPhone exactly as shown below and see whether that resolves the issue:

• Press and quickly release Volume UP button
• Press and quickly release Volume DOWN button
• Press and Hold the SIDE button until an Apple logo appears then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF). 

Axel F.

That is not how you perform a Forced ReStart on an iPhone X that the poster has. See below-

• Press and quickly release Volume UP button
• Press and quickly release Volume DOWN button
• Press and Hold the SIDE button until an Apple logo appears then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF). 

Axel F.

You should be able to hold down the power button and the volume button together to do a forced restart.