microphone stopped working...

My microphone was working just fine one day n stopped working the other
day... i have tried using the mic on another system, it works fine.. also i
have run the diagnostic test n that shows that the mic hardware is also
working. but it still cant detect any sound when i try to record. when i use
the mic to talk over yahoo massenger, only a v v low volume voice can be
heard at the other end and that too if i speak very loud. please help me with
this problem. i can not figure it out. i am using windows vista and sigma tel
high def audio codec.