Face ID Not Working When Phone is locked for a long period of time.

If you don't see a 'disabled' message but do have general issues with Face ID not working properly, explore the list of things to try in this document: If Face ID isn't working on your iPhone or iPad Pro - Apple Support

If the first 8 don't solve the issue then use #9, contact Apple.

There are several reasons why Face ID might not work on your iPhone when the phone is locked for a long period of time. Here are some things you can try to troubleshoot the issue:

1. Check if Attention Aware Features is enabled. If Attention Aware Features is turned on, your iPhone will only unlock with Face ID if you're looking at the screen. Go to Settings > Face ID & Passcode and make sure Attention Aware Features is turned off.
2. Reset Face ID. You can try resetting your Face ID settings by going to Settings > Face ID & Passcode > Reset Face ID. This will delete your existing Face ID data and let you set it up again.
3. Make sure your iPhone is running the latest software. Go to Settings > General > Software Update and make sure your iPhone is up to date.
4. Check for any physical damage. If there is any damage to the TrueDepth camera system, Face ID might not work. Contact Apple Support for assistance.
5. If none of the above steps help, try resetting your iPhone to its factory settings. Go to Settings > General > Reset > Erase All Content and Settings. This will erase all the data and settings on your iPhone, so make sure to back up your important data first.

For more information visit: If Face ID isn't working on your iPhone or iPad Pro - Apple Support