Hi mmmitchell,
This has happened to other folks, too. Here's the best way I know to fix it.
First, just to check: in iTunes, at the upper left corner of the window, look for a little "home" symbol with an up-down toggle that lets you switch between "This Computer" and "[username]'s Music" or something like that. If this is set to This Computer, and all your playlists and music are still missing, do this:
1. In iTunes > Preferences > General, turn off "Show Apple Music" and "iCloud Music Library." Then quit iTunes.
2. Find the folder Music / iTunes, and look for a document called iTunes Library.itl. Drag this file to the desktop.
3. In this same folder, you should find another folder called Previous iTunes Libraries. The files there will have names like "iTunes Library 2015-08-15.itl". Choose the one with the most recent date, and make a copy of this file (Command-D or File > Duplicate). Rename the copy to "iTunes Library.itl"
4. Drag this new file into the iTunes folder, replacing the one you just dragged to the desktop.
5. Take a deep breath and restart iTunes.
If all goes well, you should be seeing your music library as it looked before the meltdown. Please remember that if you added music or created playlists between the date of the iTunes Library backup and today, you'll need to re-import that music and re-create those playlists. (If you have trouble remembering or locating the music, there's an Applescript utility to help with this -- just post a follow-up.)
I hope this helps!