Make and model microphone or headset/microphone combo?
Have you right clicked the small speaker icon normally found in the lower right screen corner?
Go into "Recording devices" and work through the various windows, tabs, and properties. Check the other choices as well.
Be sure that all applicable drivers are up to date and that you have verified the configuration settings necessary for your computer.
Try the microphone on another computer or device. See if the problem follows the microphone.
Try another microphone on your computer. If the problem remains it could be the computer or its' sound card.