If your using iOS 7 as your profile suggests, you can also use Control Center to adjust volume. Swipe up from the bottom of the screen to display the volume slider.
Also, in the event this is a software glitch, you can try resetting your iPad by simultaneously pressing and holding the Home and Sleep/Wake buttons until you see the Apple Logo. This can take up to 15 seconds so be patient and don't release the buttons until the logo appears. Try again to see if the problem persists.