How long ago did it stop?
Have you tried either resetting your phone or deleting and then adding your account?
My yahoo email doesn't seem to be as reliable as my other ones.
Sent from my SEXY GORGEOUS AWESOME GOLD 128G iPhone 6 ️️️
Yeah, as above! Delete and re add the account. If it doesn't let you. Make sure your password is correct. You may need to log into yahoo mail on a computer and do a captcha code before you can sign in on iOS device. Yahoo can be a pain in the bum! If you can log in on the computer perfectly, I promise you readding the account will work (Settings>Mail, Contacts & Calendars.....etc)