For no reason I can see, my microphone has stopped working with Audacity.The mike itself is okay. I’ve played with the input settings and fiddled with Realtek which shows when a mike is plugged in or removed. I have made no changes to my computer and I ran a system restore.
Would it do any good to delete Audacity completely then download it again? rps 
It would do a lot of good to tell us what the microphone is, how it’s connected and a little about your computer.
How are you using it? On your third podcast and suddenly it stopped working?
Music? Voice?
You didn’t say whether or not your microphone even showed up in Audacity. Audacity checks for new connections when it starts. So plug everything up, and either restart Audacity or Transport > Rescan.
Koz
If it’s a settings problem in Audacity, reinstall Audacity using the EXE installer and enable the “Reset Preferences” checkbox half way through installation. When you launch Audacity, confirm that you want to reset.
Here is where to download the current Audacity 2.1.2 from us http://www.audacityteam.org/download/windows.
Gale
The computer is an Acer desktop running Windows 8.1. The mike is a basic good quality dynamic PA mike with a mini jack that plugs in the audio output on the front of the computer. I am recording monaural voice. It has worked without problem for 2/3 years but would not work yesterday.There have been no changes to the computer other than a System Restore and the usual Windows updates.
rps
Correction…audio INPUT. Not OUTPUT. rps
Many larger Desktop systems have a full soundcard and we may have identified the problem. They have two inputs, not only one. They can have a Pink Microphone-In and a Blue Stereo line-in.
If you cross them by accident, the microphone will drop dead. Even if you haven’t crossed them, it’s worth unplugging and replugging several times just to rule out moisture, dirt or corrosion damage.
Koz
What are the symptoms, in detail, of “not working”?
Gale
Windows sound settings has a sound meter to help you diagnose problems. Does that meter bounce when you talk?
Koz