I had to reinstall Authenticator & now a site I set up for 2-factor authentication isn't working so I can't get into the site.

You can restore from an authenticator cloud backup (assuming there was one) but make sure no accounts have been added to the newly install app. Then sign on with recovery account to do the restore.

You can recover your account credentials from your cloud account, but you must first make sure that the account you're recovering doesn't exist in the Microsoft Authenticator app. For example, if you're recovering your personal Microsoft account, you must make sure you don't have a personal Microsoft account already set up in the authenticator app. This check is important so we can be sure we're not overwriting or erasing an existing account by mistake. https://support.microsoft.com/en-us/account-billing/back-up-and-recover-account-credentials-in-the-authenticator-app-bb939936-7a8d-4e88-bc43-49bc1a700a40

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