Microsoft account suddenly says it doesn't exist, and I cannot access my OneDrive files

My personal Microsoft account suddenly stopped working.

Microsoft now says that the account does not exist when I try to sign in. However, the same account is still shown as a Microsoft account in Windows Settings, and Microsoft Office still shows it under my account.

OneDrive Personal is also still connected to this account on my PC, but it now shows “OneDrive isn't connected” and asks me to re-enter my credentials.

I have many important OneDrive files that are “Available when online” (cloud icon). I can still see the filenames in File Explorer, but I cannot open them because OneDrive is no longer connected.

I have already tried the Microsoft Account Sign-in Helper, but it could not resolve the problem.

I would like to know whether my original Microsoft account has been disabled, deleted, or had its sign-in alias changed, and how I can recover access to the original account and OneDrive data.

I do not want to create a new Microsoft account using the same email address because I am concerned that it will not be connected to my existing OneDrive data.

Please advise how I can escalate this to Microsoft Account / OneDrive support.