Hello SamL F
Hi, I'm Karl and will be happy to help you today.
First, Just so there is no misunderstanding, this is a public user-to-user support forum. We're users just like you helping other users. We do not work for Microsoft and have no access to any user account so there is nothing we can do or check from within the forum.
Once you encounter the error that the "account does not exist" that means that the email address (account) has been permanently deleted and cannot be recovered by anyone - it is permanently gone. There are no exceptions to this Microsoft policy.
No one in the forum can tell you why the account has been deleted but the most common reasons are
#1 - The account has not been accessed within the last 2 years and has been automatically deleted due to inactivity as per hte Microsoft Account Activity policy
Microsoft account activity policy
https://support.microsoft.com/en-us/topic/microsoft-account-activity-policy-7c0a9fa7-0982-b7c6-fd72-df852b63699f
or as summarized in
Check if your Microsoft account is closed
https://support.microsoft.com/en-us/account-billing/check-if-your-microsoft-account-is-closed-5ce9281c-99eb-423b-9b5c-2b69a7701dc2
#2 - Your account was hacked and the hacker added their email address to your account, made it the <primary alias> and removed your email address from the account
or
#3 - You or someone who has access to the account removed the email address from the account.
In terms of items #2 or #3, it does not matter who removed the email address from the account. The action of removing an alias email address from an account is both immediate and permanent. The email address can never be recovered. This is described at the bottom of the following Microsoft article under
<"I want to move or transfer my alias to another account to merge or consolidate accounts">
Change the email address or phone number for your Microsoft account
https://support.microsoft.com/en-us/account-billing/change-the-email-address-or-phone-number-for-your-microsoft-account-761a662d-8032-88f4-03f3-c9ba8ba0e00b
In terms of trying to create a new account using the same email address, if you try to create a new account using a deleted email address, you would encounter an error message that the email address is already in use. That is technically correct since every email address is maintained in a database so that it can be checked. What is misleading about that message is that it does not indicate that it involves a deleted email address which causes confusion