Hi Daisy. There are many types of lead. Is it HDMI? If so there is quite a good video on this page
. Go through the various steps.
Have you got any headphones available? If so the computer will probably have an audio jack plug on it. Try plugging the headphones into the jack plug and see if you get sound on the headphones. If you do get sound on the headphones the TV might well have some sort of audio input, usually two phono/RCA sockets. There might be more phono/RCA sockets so you need to find the correct ones for audio IN. You could try connecting the audio from the computer to the TV using a suitable lead such as male jack to male phono
The Amazon one I've linked is just to show you what they look like. You can buy them in any shop selling electronics/computer type stuff or through e-Shop.
A final thought. You haven't accidentally pressed the mute button on the TV remote have you?