What happens when you add the account back onto your iPhone? Does it show up in Settings > Mail, Contacts, Calendars? Does it simply not add?
If it adds, is it downloading emails? Have you tried deleting the account, hard resetting your iPhone, and trying again?
To hard reset, press and hold both Power and Home buttons for about 7 seconds until the phone turns off. This refreshes internet caches better than simply shutting the phone off.
Lastly, try going directly to Yahoo's website and see if your email works there. Sorry for so many questions, but your original inquiry is lacking information.
I have had a Yahoo email address since the first iPhone and haven't had any issues thus far. On iOS 9.3.1