Face ID not working on my iPhone X

This article has some perfect troubleshooting tips for this problem: If Face ID isn't working on your iPhone or iPad Pro - Apple Support.

If you run into more troubles with Face ID in spite of following these instructions, feel free to contact Apple Support: Official Apple Support.

Sorry, trying to fix a broken FaceID yourself is not a good idea:

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2 . If you still can't enroll your face or unlock your device with Face ID, you can take your device to an Apple Store or Apple Authorized Service Provider, or contact Apple Support about your iPhone or your iPad.

Only a trained technician should service your device. Improper repair, modification, or use of nongenuine Apple components in the laser systems might prevent the safety mechanisms from functioning properly and could cause hazardous exposure and injury to eyes or skin.

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copied from: If an alert says Face ID has been disabled on your iPhone or iPad Pro - Apple Support