I'd bet you innocuously created a new, blank version of your iTunes library, so all it "knows" is your purchase history.
Do a search on your driver for all files called "iTunes Library.itl"; you likely have two - the one you accidentally created when you noticed the problem, and one (in a separate directory) with your last known good data set. (In other words, time stamped the last time you used iTunes and it was OK).
Write down the location of the last known good iTunes Library.itl file.
Make sure your iTunes is closed.
Hold down the SHIFT key on your keyboard when you launch iTunes, This should prompt you to "Choose an iTunes Library". Select "Choose Library" and navigate to the location of your last known good iTunes Library,itl file. Select that file and continue. You SHOULD be OK.
If you can't find any additional iTunes Library.itl files, you can only recover as far back as your last backup of your iTunes directory. (To do that, just restore the iTunes Library.itl file from your last backup, and follow the same instructions as in the above paragraph).
No matter what, your music files and folder structure should be OK (check your iTunes directory for a subdirectory called "iTunes Music"; that should contain all of your .m4a files).