If you plug a USB sound device like a USB headset or other USB Sound "thing," Windows will reassign sound devices to "help you." Do you use Skype or other conferencing package? Skype is famous for reconfiguring your machine to suit itself and then not always putting it all back when it was done. Skype is going to require your built-in microphone.
You should be able to pick a specific audio input with the toolbar next to the Audacity microphone symbol. It may be set for Built-In-Microphone instead of Stereo Line-In. I'm making up those names, but you get the idea. You might need to restart Audacity if you make a change here. You "shouldn't" need to. If that fails, go into Audacity > Preferences > Devices and make sure the right thing is selected there.
Are you on a laptop? How exactly did you connect your mixer to your computer? Generally, plugging the mixer Line-Out into the computer Mic-In doesn't work all that well because of serious mismatches.
Scroll down to "Connections."http://www.kozco.com/tech/audacity/wind ... rding.html