My Face ID stopped working after I installed iOS 14.6

If you still need help, reset Face ID, then set it up again. Go to Settings > Face ID & Passcode and tap Reset Face ID. Then tap Set up Face ID to set it up again. If you can't enroll your face, take your device to an Apple Retail Store or Apple Authorized Service Provider or contact Apple Support.

If Face ID isn't working on your iPhone or iPad Pro - Apple ...

Before updating to the latest iOS 14.6 as told in your post what was your earlier iOS version. Then you can look into the iOS changes that have happened between the earlier version on your iPhone and this new version that you have updated now. 

This will give you a broader idea of the issue if it is directly linked to the iOS version or is triggered by something else.