Handoff not working on iPhone X and iOS11

Start here:

System requirements for Continuity on Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support

and here

Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support

Ok, which app are you trying to use Handoff?

Example: Apps that work with Handoff include Mail, Maps, Safari, Reminders, Calendar, Contacts, Pages, Numbers, Keynote, and numerous third-party apps.

Which one did you use?

Check list here:

Use Handoff with any Mac, iPhone, iPad, iPod touch, Apple Watch that meets the Continuity system requirements. Handoff works when your devices are near each other and set up as follows:

• Each device is signed in to iCloud with the same Apple ID.
  To see the Apple ID used by Apple Watch, open the Apple Watch app on your iPhone, then go to General > Apple ID.
• Each device has Bluetooth turned on.
• Each Mac, iPhone, iPad, or iPod touch has Wi-Fi turned on.
• Each device has Handoff turned on:
  • On Mac, choose Apple menu () > System Preferences, then click General. Select “Allow Handoff between this Mac and your iCloud devices.”
  • On iPhone, iPad, or iPod touch, go to Settings > General > Handoff, then turn on Handoff.
  • On Apple Watch, open the Apple Watch app on your iPhone, then tap General and turn on Enable Handoff.
    Apple Watch supports handing off from watch to iPhone only.

Go through this tips again carefully: Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support

Ok. Just to complete this discussion, signing out of iCloud on my iPhone X and iMac and restarting both, and signing back in to both, got Handoff working correctly...

Thanks! I couldn't get Handoff to work between my iPhone and iPad on iOS 11.3, and although it was a pain, that worked.

Try on your X

Restart / Force Restart

https://support.apple.com/en-us/HT201559

Here: Genius Bar Reservation and Apple Support Options - Apple

Without details on your operating systems (macOS & iOS) and details on what you have done, we can't help you.

Up to date? We need to know the version exactly.

What troubleshooting steps you have done? Details please.

Glad it's working for you, cheers.

it was when I signed out of iCloud on all devices to fix a different issue that Handoff Broke!