Are you referring to iOS5 when you talk about the 'upgrade'? If so, are you using IMAP or POP as your email protocol?
But, as a quick possible fix (only 'possible', not certain) try powering the iPad down completely by pressing and holding the Power button until the slider control appears, then accept the offer to switch off. When the screen is completely blank, press the Power button again until the White Apple logo appears.
If that fails, try closing the Mail completely in the multi-task bar. Open the multi-task bar by double pressing the Home button. Now press and hold any icon in the bar until they all start to jiggle. Now touch the upper left hand corner of the Mail app icon to close it completely. Now press the Home button again to close the multi-task bar and see if the problem is fixed...
Let us know how you get on......
Tim