I suspect you mean the Mail App icon on your home screen (or dock). Have you tried using lock button to reveal shut down slider and shut down the iPad. Now restart the iPad by holding dame button Lock Button). If Mail icon hasn't returned, next simultaneously hold down Lock button and Home button and repeat the procedure. If problem not resolved you will have to do an iPad Restore while connected to your computer (iTunes). The following directly from iPad User Guide for iOS 5 (free from iBook bookstore):
Restore iPad files and settings: *Connect iPad to the computer you normally sync with, select iPad in the iTunes window, and click Restore in the Summary pane.
