Ensure that your microphone is plugged in and configured correctly, this site will help you check if your browser is able to detect your audio: https://www.onlinemictest.com/.
If the website above does not work, it is possible that the microphone is not being recognized by the computer.
Start by figuring out which operating system you are using by visiting https://www.whatismybrowser.com.
If you are using Windows 7, there may be extra steps you need to take to configure your microphone. This link has step by step instructions: https://www.onlinemictest.com/microphone-settings/windows-7/.