Sorry to hear of your problem - but sadly so many people do not make proper back ups.
I have several suggestions:
1. As you launch iTunes, keep the shift key pressed. A dialog should appear asking if you want to create a new library, or choose another. You could try switching, via windows explorer, to the old library. If this works, next time you launch, this will be the default library.
If not you could, in iTunes Music, click on File, then "add folder to the library" then find the old iTunes Music folder (not the library) and enter it.
Do come back if you like.