What computer? Laptop or desktop if Desktop how are the speakers connected or are you using headphones. Go to bottom task bar and look for speaker symbol Click on it ans ensure the Mute Box is not ticked. Still no sound? Go to Start/Programs/Accessories/ Entertainment/Volume control/Make sure Mute is not ticked- check also the various sliders.
Still no sounds? Come back for more with detail of computer make/model.
How about the Windows sounds, at welcome and shut down. As others said, check Control Panel>Sounds and Audio devices and alternatively depends on which Windows version you have Start/Programs/Accessories/ Entertainment/Volume control/Make sure Mute is not ticked-
Now ........ If there's no sound even there and the Taskbar volume control is turned to Max, then it is a hardware fault.
At its simplest it could be a broken wire in a speaker lead, or perhaps you have them plugged into the wrong socket at the back.
If you have some older speakers or can borrow some to try..... Its a long trail but needs checking step by step