Double check and make sure they are plugged in correctly (like output and input connected in the right place) and add the speakers as your default in the sound manager. (usually its in the taskbar)
They are portable/external speakers correct?
Here are some tips I found from my portable speakers guide. For Mac and Windows.
Q. Why can't I hear any sound from my speakers?
Make sure that an auxiliary cable is not plugged into the auxiliary port on the back of the Logitech® Z-5 speakers.
Verify that your speakers are set to be the default audio output device:
----- On Windows® XP:
Go to Control Panel > Sounds and Audio Devices, and display the Audio tab.
In the Sound playback area, choose "SPEAKERNAME" from the Default Device list.
Click Apply, then click OK.
----- On Windows Vista:
Go to Control Panel > Sounds, and display the Playback Devices tab.
On the Playback tab, choose "SPEAKERNAME", and click Set Default.
Really, there could be a ton of problems with it. You just gotta be more specific, First and foremost are you plugging it in the right socket...? Lol. It could be the simplest things, I know one of the reasons was that the cable wire tore inside but was still covered by the rubber so i couldn't see it. Good luck,