One or more of the following may occur:

Mail app unexpectedly quits when trying to access mail
or
iPhone or iPod touch displays "Home" screen when trying to access mail
or
Mail account on the iPhone or iPod touch shows blank account title/description in Settings > Mail, Contacts, Calendars screen as shown here:

Related posts:

1. MobileMe: Features that work with one account at a time on iPhone, iPad, or iPod touch
2. iPhone and iPod touch: What to do when applications close unexpectedly while in use
3. iPhone and iPod touch: If IMAP Mail folders are not visible
4. iOS 4: Unable to view message body preview or mail receives blank messages
5. Overview of syncing iPhone or iPod touch with your computer