The situation matches having two different Microsoft accounts using the same email address: a personal Microsoft account and a work/school account, or two separate Microsoft accounts that share the same sign-in address. When signing in on the PC, the newer account is being selected, so only that mailbox appears.
To get back to the original mailbox on the PC, use one of these approaches:
- Choose the correct account at sign-in
- When the "Which account do you want to use?" prompt appears, select the Personal account (or the one that corresponds to the original Outlook.com mailbox) instead of the work/school or newly created account.
- Sign in with the same credentials that are currently working on the phone.
- Rename the newer/personal Microsoft account so the prompt goes away
If the same email address is being used for two accounts and the prompt keeps appearing:
- Go to the Manage how you sign in section for the personal Microsoft account.
- Add a new email address (alias) or use an existing one.
- Set that new address as Make primary.
- After that, sign out and sign back in on the PC, and choose the account that corresponds to the original Outlook.com mailbox.
This separates the sign-in identities so the PC can sign in to the correct mailbox again.
If the old mailbox was removed as an alias from a Microsoft account, it is permanently deleted and cannot be reattached to any Microsoft account.
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