So I just started using Audacity to record some of my own singing onto a karaoke track. The first time I tried it, it worked out just fine, and now when I record my singing with the karaoke track, it won’t record my singing, and my output ends up being only the karaoke track. What am I doing wrong? 
Are you still recording your microphone? Check the Device Toolbar.
http://manual.audacityteam.org/o/man/device_toolbar.html
If you unplugged a USB microphone and then plugged it in again, Audacity may forget about it. Restart Audacity or Transport > Rescan…
Make sure you’re still set up for Overdubbing. If you like to record Internet Audio, those settings can seriously mess up Overdubbing.
Right-click inside the red recording meters > Start Monitoring. Sing or talk into the microphone. If you don’t get the bouncing red sound meter, then your sound channel is dead.
How far did you get?
Overdubbing setup instructions are here. One step very early on is the ability to make a simple, non-overdubbing recording before you start being fancy. If you can’t do that, you could be there for weeks messing with settings.
http://manual.audacityteam.org/o/man/tutorial_recording_multi_track_overdubs.html
Koz