Whenever you are unable to access the yahoo account on your iphone. It can be due to the several reasons. visit askprob community to get the problems resolved.
1. Mail application issue.
2. Account settings issue.
3. Cookies or cache problem.
3. Settings problem.
YOu should clean the cache of your app, and then try to load the mail again. somehow, if you don't get your email working. you need to reset the yahoo account on your phone.
For more information: you may visit: how to fix yahoo mail not working on iphone./