e machines 420 NO SOUND?

i recently took my computer to pc world because the it needed a clean up (remove dust). when i got the computer back and turned it on i lost the sound, friends have suggested checking the sound card but i dont know what it is or where to find it. others have suggested as it is still under warranty taking it back to pc world. as a last resort maybe ? can anybody out there assist. ive already checked the sound features but still no joy.