You were asked to register for multi-factor authentication, or you got a new phone and MFA prompts no longer arrive.
1. Install Microsoft Authenticator from the App Store or Google Play. 2. On a computer, open https://aka.ms/mfasetup and sign in. 3. Choose Add method → Authenticator app → Next → scan the QR code with the app. 4. Approve the test notification. 5. New phone: remove the old device under Security info, then repeat the steps. If you cannot sign in at all, submit a request — an agent will reset your MFA methods after identity verification.