Configuring preferences and options in Outlook.com
Microsoft has been advertising the change for well over a year and had sent at least two emails to every single user
They are migrating all users and once an account has been migrated there is no going back. The migration can take 2-4 days after which time your data should return. If it does not Unless you made the back up that Microsoft recommended when you signed up for the email address, the data is gone because, like you, the online servers are not backed up