Can't hear sound from computer

We have tried a few things and we still can't hear sounds from the computer. When we plug in headphones, we hear sound but when connected to the monitor, no sound. Any suggestions?

When connected to the monitor? Do you mean your monitor has built in speakers? When you say you plug in headphones, do you mean you plug in headphones into the same jack you plug your speakers/monitor into and it works? Or, are you using a different jack. Have you checked that the volume/mute controls on the monitor are adjusted appropriately? Have you tried plugging the monitor into the same jack you plugged the headphones into? Barring that, buy a set of external speakers to solve your problem. Apparently the speakers in the monitor are bad.

Any suggestions? Yes. The monitor's setup buttons might control the monitor's built in speakers. The speakers might have been accidentally turned off in the setup menus, or the volume turned to the lowest so they are basically silent.