Hello ! I'm Aymeric, an independent advisor, and I'm here to help !
On the left, you can see the e-mail accounts that have been added to Outlook.
If it's not here, then the account in question is no longer present in Outlook. I therefore invite you to add it again by going to Outlook settings > Accounts > Add account.
Then, follow the steps to add your account back.
Let me know if you have any other question !