iTunes combined half of my music into one album. How do I undo this?

Some questions:

1. Do you know exactly which properties you overwrote for too many tracks at the same time? I'm guessing Album, Album Artist and Artist, but it could be more.
2. Artists A-P, Albums A-P, Songs A-P?
3. Under Edit > Preferences > Advanced is Keep organized selected, in other words have the files and folders inside your media folder been reogranized to reflect this mistaken edit.
4. What is the most recent dated file in the Previous iTunes Libraries folder inside your main iTunes folder?

I'll assume you don't have a backup to restore, or you'd be doing that already. I have a script called ExportImport that can be adapted to export the corrupted properties from a backup of the database found in the Previous iTunes Libraries folder to a text file. (See Open a different iTunes Library file or create a new one - Apple Support for details of how to switch between libraries.) You can edit the script in Notepad or Notepad++ and comment out properties that you don't need to export. Select the files that you think have been damaged when looking at the old library (question 2) or export everything in the Songs list if you cannot be sure. After creating your export file you switch back to your original .itl file in the main iTunes folder and drag the exported data onto the script to import it, which should repair the damage. If Keep organized was off, and the files haven't been moved, then before you connect to the older database file rename the media folder as NOT iTunes Media so that iTunes cannot read the updated tags. After the export close iTunes and rename the media folder back to iTunes Media before switching to the original library and importing the correct properties.

See Backup your iTunes for Windows library with SyncToy to preserve your library in future.

tt2