Hello.
To help me narrow this down, could you let me know exactly which Windows build you are running and when you first noticed the login option disappear?
For those instances where you are locked out entirely, booting into Safe Mode may allow the hidden built-in Administrator account to appear so you can apply these fixes. Applying these specific registry adjustments should bring back the flexibility you need to log in with any valid credential on your network.
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Best regards,
Scott Nguyen