The most likely explanation is that it’s due to the confusing nature of the mute button on the iPad (we get a lot of mails about this). Have a look at the switch next to the volume controls. If you can see a small orange dot inside then the iPad is muted. Switch it the other way and it should work. It’s confusing because lots of apps (especially music/video players) ignore the mute button and make noise anyway, so lots of people don’t realise their iPad is muted.
On newer devices without a mute button, swipe up from the bottom of your home screen to get to a simplified settings menu. On it, you should see a bell icon with a line through it, which mutes the device. Check that this is disabled.
If that doesn’t help, turning off your device (by holding down the power button until “Slide to power off” appears) can also solve this problem.