My IPad has stopped "ding" when a new email appears. How to fix?

Have you got email notifications turned 'on' in Settings > General > Sounds > New Mail ? If you have and you still don't get sound then is the volume turned down and/or do you have notifications muted ?

Depending on what you've got Settings > General > Use Side Switch To set to (mute or rotation lock), then you can mute notifications by the switch on the right hand side of the iPad, or via the taskbar : double-click the home button; slide from the left; and it's the icon far left; press home again to exit the taskbar. The function that isn't on the side switch is set via the taskbar instead : http://support.apple.com/kb/HT4085