Hm, assuming the Mail program is working.
In landscape mode you'll see a button in the top left corner. It's got a slightly pointy end on the left. Tap this as many times as it takes to get back to the Inbox list.
In portrait mode there is a slightly different button in the top left corner. When you tap this a list comes down. In the top left corner of the list is the same back buttons you find in landscape mode.
If you aren't using Mail, or the buttons aren't there, I'm at a loss. Though powering the iPad completely off and then on again sometimes clears up weird app behavior.