When I click on to Mail (Hotmail UK), a brief flashing message tells me “firstname.lastname@example.org is not available”. I am available. I cannot access any of my recent email messages or newsletters. What has gone wrong?
Hotmail is dead for a long time, as well as Live! services. Use Outlook instead.
When was the last time you logged in?
Accounts are automatically deleted if you didn't sign in within 365 days.
Also, did you upgrade to Outlook.com? Did you create an outlook.com email address?
Maybe you turned the hotmail.co.uk address into an alias in which case you will have to sign in using your new primary address. You cannot sign in with aliases.
sign in to your account via https://mail.live.com/ or https://www.outlook.com to re-enable it.