There have been randomly persistent issues with yahoo for months. They are in the process of 'upgrading' their mail and I'm also going to guess upgrading servers too. I've experienced periods of outages on both my yahoo accounts.
There have also been cases of people needing to delete and re add their yahoo info after upgrading the iOS.
Unfortunately, the issue is usually on Yahoo's end. Your choices are to wait it out or find a work around. When mine woulnd't work I simply accessed it another way. When my account wouldn't work on my computer, I accessed it on my iPad and vice versa. When it wouldn't work in the mail app, I accessed it via safari.
Yes, it's not you, it's them. Their servers are either overloaded, buggy, or both, and they randomly disconnect all the time.
I haven't been able to find any workaround.
That and the better features of gMail finally convinced me to just switch (this also cut loose from years of slowly accumulating spam lists I was on). I check Yahoo once or twice a week via the web interface, in case anyone didn't get the message.