Try restarting the iPad. Hold down the power button until you see the power off slider. Slide off. Wait until the iPad shuts completely down, then start it again by pressing the power button for a couple seconds.
If it's a glitch with the Mail app, that should help.
If there is a back button at the top left of the email list, tap it and see what happens. The button can say Mailboxes, or it might say something else. If the choices are not obvious, say something.
When you are looking at the All Inboxes and account mailboxes, there is an Edit button at the top right of the list. Tap this and you can choose what special mailbox/folder are visible.
If none of this works, got to Settings > Mail, Contacts, Calendars and make sure your email accounts are still there. Open each account, and if the option exists make sure Mail is turned on.
If none of this helps, please give us more details on what is going on.
Good luck.