Try resetting it. Hold down on the sleep button and the home button at the same time for about 10 seconds until the Apple logo appears on the screen, ignore the red slider, let go of the buttons and see if the iPad starts up.
Try resetting your iPad by pressing the home and sleep buttons until you see the Apple logo, ignoring the slider. Takes about 5-15 secs of button holding and you won't lose any data or settings.
If that doesn't work (probably won't in your case but it's quick), you'll need to restore via iTunes... http://support.apple.com/kb/HT1414