So, I think iOS 12 just killed my iPhone X. This folks, is why you don't run betas!
@rosemaryorchard forcing a reboot has helped with mine. I've seen that a couple of times. Press in sequence (not simultaneously) volume up, volume down, then press and hold the sleep/wake/Siri button until you see the Apple logo. HTH.
@Lioncourt Well all buttons and the touchscreen stopped working. The screen went black which was more than a tad scary. However I plugged it into iTunes and it was recognised, waited ~5 minutes and the screen lit up and went crazy doing everything I'd tried to do before. A reboot was done and it all seems fine again... freaky, huh?