When I checked my Yahoo Mail account today, it hung after I pressed enter, then eventually displayed a message to the effect that there was a problem and would I like to switch off the beta version of Yahoo Mail. I didn't know that I was using a beta version and clicked Yes. Then I entered Yahoo Mail and found a much inferior display format to that to which I was accustomed. This must be the original format for Yahoo Mail, which most people never see because there is no way to switch to it unless you receive the special error page that I received. What Yahoo refers to as the beta version is the current default.
I have been unable to find any information in Yahoo Help or elsewhere on the web about how to undo what I have done. Buried deep in Yahoo Help is old information about the beta version, which advises that one can toggle back and forth between the original and the beta by using a link on the Yahoo Mail Welcome page. Such a link no longer exists. I filled out a form in Yahoo Help, and received in response an email inviting me to search for the answer in Yahoo Help, and if that fails to fill out a form requesting support, which is what I already had done.