by the sounds you have uninstalled your audio device driver or have selected the wrong audio device.
my best bet would be driver so I will work with the information I have.
firstly to determine it's the driver do as follows.
1. click start
2. right-click on computer
3. click properties
5. click device manager
check for any unrecognized devices (usually have a yellow asterisk next to them, and are listed under other devices)
if this is the case. go to your computers manufacturers site. search under support for drivers, and select your model. under audio should be a driver for your computers device. download and install it.
if this is not the case could you provide some extra information to help me determine the problem. such as computer brand, device name etc.
wait for your reply :)