Ugh that can be a hardware problem or software. On the hardware side double check your connections
A. reseat the cables, maybe even try a different port (ie front or back)
B. make sure there is not a connection on the headset for the mic that has come loose
C. make sure any in line volume control does not have the mic turned to mute
D. try the mic/headset on another computer or audio device,
E. try another mic to see if its detected
Next you may want to look into System Properties--Device Manager and see if there is an error message. Beyond that I am not help with the software side. Good luck.