I wrote before on How to solve stuck iPad keeping asking for Exchange password: reset your iPad (or iPod/iPhone if it happens with those).
Actually, you should first verify if your Outlook Web Access still works with the same password, and if it does, then try to restart your iPad/iPod/iPhone, and if that fails, reset it:
Restarting your device
Press and hold the Sleep/Wake button for a few seconds until the red “slide to power off” slider appears, and then slide the slider.
Press and hold the Sleep/Wake button until the Apple logo appears.
Note: Reset your device only if you are unable to restart it.
Resetting your device
Press and hold the Sleep/Wake button and the Home button together for at least ten seconds, until the Apple logo appears.
–jeroen
via iPhone, iPad, iPod touch: Turning off and on (restarting) and resetting.