Most likely you've managed to mute the iPad. Normally this only affects systems sounds, alerts, notifications and such, but sometimes app developers code their apps incorrectly and they are also muted.
In Settings > General > Use Side Switch to: you can either set it to Mute or Screen lock. This is the switch next to the volume rocker. If it is set to Mute try turning the switch off. If it is set to set to Lock Screen, then you need to turn Mute off in the multitask bar.
To get to the multitask bar double tap the home button. Drag your finger left to right in the bar to get the controls, where you will see either a Mute icon or Screen Lock icon; the opposite of whatever the switch is set to.