Rear 3.5mm Jack Microphone not working on Compaq CQ70 Laptop

Hi there I am in desperate need of help,
I have recently upgraded my computer to windows 8 and since doing so the built in microphone has stopped working on my computer but it was working before when I search for the device it just comes up with its not plugged in! I am really confused on how to solve this problem. I have tried everything I can think of so some help would be very grateful please.
Many Thanks
Cassie