I forgot my username or email address

You can attempt to remember your username on your own:

If you correctly entered your first and last name when registering and linked your phone number or an alternate email address to your account, Yandex can find all the usernames associated with that data. To do this, open the login recovery page and follow the instructions.

Automatic login search only works if you entered your first and last name correctly. If the login cannot be found, perhaps you signed up using another last name or your first name in another form (such as “Sam”, not “Samantha”).
Go to the Restore access page. If you have recently accessed your account rom the computer or browser you're currently using, you'll see a username suggestion appear under the Username or email field.
If you used this address to communicate with your friends, family or colleagues, try to contact them, for example, through social networks. Maybe they will remember your address or find a message from you in their inboxes and can tell you what address it was sent from.