If you use the built in Mail app, delete the Yahoo account and set it up again. If it's the app, delete it from your iPad, restart your iPad, reset it (press and hold Home and Power button until the Apple logo appears, then let your iPad power up on it's own). Install the app once more.
Clear history, cookies and data in Settings - Safari.
Force close the app with the account: double tap the Home button to reveal the multitask bar. In iOS 6 tap and hold the app icon until it starts to wiggle and delete it. In iOS 7 slide the picture above the app icon towards the top of the screen.