Computerworld - https://www.computerworld.com/article/3238468/apple-ios/the-new-apple-iphone-x-troubleshooting-guide.html#tk.rss_all
Apple’s new iPhone isn’t having too many problems, but everybody needs to know what to do if a problem strikes. The disappearance of the Home button means some traditional troubleshooting techniques need to change, even while we need to get used to using all the new gestures.
Force restart iPhone X
In many cases a force restart of an iPhone X will solve any problems you may meet, but because iPhone X lacks a Home button, you need to follow these steps to make this happen:
Press and quickly release the Volume Up button Press and quickly release the Volume Down button