Check the foillowing:
1. That your microphone is the correct type to use with the input. There are several different kinds of microphones, and each needs different kinds of electronics to work:
- Crystal - just needs the D/A converter on a line input.
- Dynamic - needs a preamp
- Condenser - needs a power source and a preamp
2. That the jack is actually connected to something. Often I have found the front panel mic jack disconnected.
3. That your microphone device driver is installed.
4. That the microphone channel strip on the audio mixer (found in Control Panel under Audio Devices) has the fader for the microphone set at the correct level (mine defaulted to being set all the way down when Windows was installed).