Yahoo's servers have been experiencing problems this last week. I thought the ETA for a fix was today, but that just might be a rumor.
You could try closing the mail app and resetting for now:
Close all open apps by double-tapping the home button, then swiping up and off the screen with the app window (not the smaller icon).
Hold down the home button along with the sleep/wake button until you see the apple, then let go.