Ah, I bet I know the solution to this! It's probably just changed your input device on ya. But first thing's first, make sure your mic's plugged in the right input slot, most of the time it'll show a microphone symbol, or if it's color coded, pink is the standard color for microphone.
Ok, go start > control pannel > sounds and audio devices > click the voice tab > under "voice recording" click the "volume" button.
Now, make sure you have your microphone selected, then try turning up the volume bar to half ways or more if you'd like. Then test! there's a couple ways. There's the sound recorder, start > all programs > accessories > entertainment > sound recorder > press "record" and try talking into the mic.
If that fails? then your mic might be called "line in" if that also fails? test the mic onna buds computer cause the mic itself could be broken. If that works? well.. you can try fiddling with it on your computer and if any of the inputs fail n' such then you could have a busted soundcard in your comp, or wose, a mobo with a dead pci slot if your soundcard's a pci plugin.
Later-A-Much, and LONG LIVE THE D!