Have you just pressed the mute button. On my laptop (a Dell) there is a button which turns off the speakers and if you have pressed that, it shows up with a cross or a line through the speaker icon. On my laptop, this button is infront of the keyboard, on a bar - there are three buttons that control my speakers - they all have a speaker icon on them - the other two are for controlling the volume but the mute one is on the left of the three and has a speaker symbol with a cross through it.