You have accidentally activated the Mute function, which suppresses system sounds. It's controlled by either the hard switch next to the volume control or the soft switch at the far left of the multi-task bar. Which of these switches controls the Mute function can be changed in the 'Settings' menu.
I had the same problem. If you still do, be sure to check the Mute button above the Volume button on the side of the iPad, and
make sure it has not been turned off. If it is off, turn it on and you should be all set.