Well, A new microphone maybe?
Heh.
There can be many reasons as so why it's not working - but don't assume that is it broken - despite my humour
.
TRY: 1. Go to 'Control Pannel'
2. Double Click on 'Sounds and Audio Devices'
3. Click on the 'Voice' tab
4. Check the right microphone is selected - Windows sometimes alters settings on it's own accord.
ALSO: Skype can also cut out as well - did you try using your mic using a different programme? Audacity will pick up any microphone plugged into your machine - it's free to download, but for safety, get it from cnet:
www.cnet.com
Once installed, run audacity, hit record and start talking. Make sure the drop down box in the top right corner has 'Mic' selected or it won't work.
If none of the above works The microphone may be broken - but seek advice from somebody who's computer literate - if they can't get it to work, then it's highly likely the microphone is broken.