Test your microphone/headset out by going to Control Panel->Sound and Audio Devices->Test Hardware. When it comes to the point where you have to read out an assigned paragraph and listen to the input, keep an ear out. If you don't hear talking, that means the hardware itself is probably broken. If it is a hardware issue, my guess is that you have a headset with a volume/mute control somewhere in the cord, and the cable has come unsoldered or unseated somewhere in that volume control.
In that case, unless you're a master with a soldering iron and blowtorch, I would not recommend fixing it yourself. Just buy a new headset.