Someone with administrative privileges on the site un-checked the "Use Email for Sign In" option the Site Settings. If you know your user id, you can login with that and go to Administration > Site Settings > Security and check the "Use Email for Sign In" option. If you don't know your user id, you will need to run the following on your database:
UPDATE mp_Sites SET UseEmailForLogin = '1' WHERE SiteID = '1'
Please ensure the SiteID in the SQL statement above is correct.