Hello Jingles2,
The first thing we would suggest would be to power cycle the iPhone by pressing and holding either volume button and the side button until the power-off slider appears. Drag the slider and wait 30 seconds for the device to turn off. While simple, this can refresh the iOS, apps and connections. More information is available here: Restart your iPhone
If the issue remains, go ahead and work through these steps:
- Check for updates
- Check your Face ID settings
- Check that nothing is covering the TrueDepth camera
- Make sure that your face isn't blocked
- Make sure that you're facing the TrueDepth camera
- Restart your device
- Add an alternative appearance
- Reset Face ID
- Get Service
Details for all of these steps can be found here: If Face ID isn't working on your iPhone or iPad Pro - Apple Support
If you see the message, "Face ID has been disabled", take a look here: If an alert says Face ID has been disabled on your iPhone or iPad Pro - Apple Support
Have a good day.