This has happened to me today, I have a feeling the account has been deleted as I hadn't used it in a couple of weeks. I have raised a ticket but not had a response as of yet.
I was working fine up until today. I only got the problem after changing my cpanel password. I changed it and I'm 100% sure I know I got it correct because you have to enter it twice. I logged out and then tried to login straight away again and it's having none of it. and the password reset...