I got a request from Yahooto manage all my settings. i rejected them all.
Since then, my contacts on my Yahoo mail account won't display ( they come up for a nanosecond, then disappear), and I've tried to send an email but the contact won't display in the sent field.
Despite this page emanating from a company that wants to get US visitors to its site to use their premium rate number for support, the actual information on it looks sound enough to me. So it might possibly help you:
https://www.contact-customersupport.com/yahoo-mail-contact-list/