if you are using win 8 follow these steps
search in start screen for manage audio devices and under settings click manage audio devices
and when a window opens click the recording tab and see whether your microphone is with a green tick
if its ok theres no problem then try speaking to the microphone and see whether near the microphone the green cells are coming up when you speak if the cells dont come up double click the mic icon and when the properties comes click on the levels tab and see the microphone volume is full