I have seen this happen due to a macOS bug with some Macs. Do you only have a single user account on this Mac?
If so, then the only way to recover is by creating a new admin user account so that the original user account reappears. In order to create a new admin user account you need to trick macOS into running Setup Assistant again so it creates a new admin user account for you. Try using the instructions in this link:
https://discussions.apple.com/docs/DOC-250002676
If this fixes the issue, then you will need to retain the new admin user account which most likely will not appear on the login screen since the bug seems to make the last created account disappear, but your original admin user account should reappear.