Edit: forgot to put in, check to see if you have microphone muted in Windows.
1. Make sure you have it plugged in correctly
2. Make sure the drivers for your audio hardware are installed
3. Could be your audio in jack went back, do you have one on the front or back you can try? Most systems have audio in/out on both front and back of the PC, try the other one.
4. Test your microphone at another system it could be bad.