Several solutions that might help.
Simple ones first! Sometimes it's easy to accidentally hit the mute switch and cut off quite a lot of sounds.
Also, remember that the iPad has independent volume for headphones and speaker. You can have the speaker set to zero and the headphones set to full volume independently. So check, if you've just switched from one to the other, that the volume control doesn't need to be reset.
If these don't work, we'll suggest some more.
Tim