Did you by any chance download any of other Apple apps such as Music, TV or Devices from the Microsoft App store to your PC?
If so, any one of them that are installed will disable access to your library on iTunes and most features of the iTunes application limiting it to Podcasts and Books only.
You can uninstall them if you have them to return iTunes to its full function, or download the Apple Music App instead if you want to keep any other apps you have. The Apple Music app should load your existing iTunes Library automatically once installed.
Outside of that, the library may be corrupt or had an issue loading and iTunes will then generate a new empty one.
You can try getting back to your old library by closing iTunes and, re-opening it with the shift key held down. It will then prompt you to choose a library to load. Navigating to the folder where the music is and selecting the iTunesLibrary.itl file there should open it if it's still functional.
If the opening process issues an error or otherwise won't load, see the following user tip.
click here ➜ Empty/corrupt iTunes library after upgrade - Apple Community