You have not given us much to go on.
Is it a laptop, or a PC with a separate monitor?
Some TFT (flat screen) monitors do have small speakers built in. Is this your case?
If so, have they ever worked?
Have you changed anything recently (hardware or software) that may have caused them to stop working?
I have a flat screen monitor with small speakers built in. If I want to use them I have to plug in a small cable into my monitor, and then plug the other end into the sound card on the back of my PC.
Have you go this small cable, or have you never needed it?
Do you know how your monitor is connected to your PC. Most monitors either use a VGA cable or a DVI cable, but I do not believe either of them carry sound.