If your iPad starts to suddenly shut off even when you know it’s charged the first thing to do it to reset it by holding down the on/off switch and Home button at the same time until the screen blacks out. Make sure you don’t use the “Slide to Power Off” text though because you are resetting it, not turning it off.
If resetting your iPad doesn’t fix the problem, you might have to restore it. For that you just connect to iTunes, select your iPad and then choose the Summary tab. You will find the directions for the restore there and choose “Yes” to backup so you don’t lose any data.