What I'd do, after re-adding the email, is manually fix the settings to the right servers.
One other big change is the discontinuation of Ad Free Mail, which is replaced with a new plan called Yahoo Mail Pro.
If so then you may need to update your i Phone or backup and restore the i Phone software. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
Would you like to answer one of these unanswered questions instead?
See also: 5 Ways To Fix Unresponsive Snapchat On IPhone When the app is unresponsive, you will have to force it to close, and open it again. To do this, tap on the icon of the app twice, and when an X appears, click on it. Go to the App Store, find a Yahoo Mail app, download and install it on your device.
To do this, press Home button two times, and you will get the list of recently used apps, including Yahoo.