My ipad has locked-up, it has frozen while running an app, and will not respond to any button, what do I do?

My ipad has locked-up, it has frozen while running an app, and will not respond to any button, what do I do?

It simply stays on the same screen image, and does not respond to anything. When I plug it into itunes, there is no indication of its presence.

The ipad's clock has also stopped.

eek