well it would help if we had more information... did you build this computer yourself? Are you plugging it into the front jack? did the microphone work before and then stop working? did you just upgrade to windows 7 and did that cause it to stop? did your microphone never work?
Self build: If you built the computer yourself and are plugging it into a front mic jack, it could be that you forgot to connect the front panel jack to the motherboard. If that's the case, try that and see if it works.
Plugged into front jack: If the above doesn't work, or not the case, try plugging into the mic jack in the back of the computer.
If it worked before and stopped: Maybe you rolled over with a chair/crimped a wire/ broke the mic?
did it stop working on an upgrade to Windows 7: if this is the case, it may simply be that you did not install all necessary drivers for your audio for Windows 7. you can check for drivers from your motherboard manufacturer's website.
Also: if you installed a soundcard yourself, make sure it's seated properly. And make sure it's plugged into the correct jack. Typically the mic jack is pink.
Good luck, I hope one of these works. Next time please provide more information in your question, so it's easier to figure out