have you been experimenting with the access control feature, or possibly, have you recently tried editing the default audio view scheme?
For access control:
Click on "View" in the top toolbar, then again on "Access Control". If it's enabled, disable it. Does your music return?
For the Audio view scheme:
right click on "Audio" and click on "Edit view scheme". Step 4 of this dialogue box should read Media Type: Audio with possibly some "~sort=[blah],[blah]" stuff afterwards. If it does, then that's not the problem so click on the cancel button. if it does not, could you let us know what "Step 4" contains?
if it's neither of these two, then I think it's likely that you have a corrupt library in which case you'll need to restore your library from a backup made before the corruption set in. If you haven't made library backups, don't fret as MC makes its own automatic backups for you. One step at a time though, let's deal with other possibilities before rolling back the library itself.
-marko.