Hi,
1. What happens when you plug in the speakers?
2. Do you get any error message?
3. Have you installed the software for the speaker?
I would also suggest you to check if the speaker works on another computer.
Method 1:
No sound in Windows
http://windows.microsoft.com/en-US/windows/help/no-sound-in-windows
Method 2:
I would also suggest you to go through the steps mentioned in the link and check.
Connect a microphone, music player, or other audio device to your computer
Method 3:
I would also suggest you to go through the steps from the Logitech website which talks about installing the speaker.
Using both connections with the Z120 speakers
You can also contact Logitech if you need further assistance.
Contact for Stereo Speakers Z120
http://www.logitech.com/en-us/439/8114?tabs=1,3,2,4,5&hub=1&subsection=phone§ion=contact
Hope this helps and keep us posted.