I'm just experiencing the same problem, my account (which I had for the past 10 years) has suddenly gone missing since 8 Jan 2021. One day I woke up and saw a "login failed" message on my phone, and when I tried to log in I got this message:
Sorry, we don't recognise this email address.
I contacted Yahoo customer service by all available means (phone, live chat, email) but they couldn't help as my account was not even accessible from their system.
I was told that there is nothing can be done at this stage, and that I have to wait and hope for the problem to be solved by itself!
I'm sort of devastated as I had that email address for almost 10 years and had all my uni, work, bills, accounts, subscriptions, personal details, banking and almost my entire life in that account.
Not sure what else I can do. If anyone has any information or had similiar issue that would be really appreciated to share with others.