I had the same problem and figured out how to fix it.
My old PC was Windows 2007, where my iTunes folder and library files were in C:/Users/Mike/My Music/Music
My new PC is Windows 10. It takes the first 5 characters of your online Microsoft account (required) as your computer user name, so in my new PC the iTunes folder and library were in C:/Users/namas/Music (to find the path in Windows 10, open up file explorer, right click on "Music" and select "properties, then look at the "Location" tab)
The solution:
In my new PC, I double clicked on the file "iTunes Music Library.xml" it opened in WordPad as a text document with all the XML code. I could see my playlists associated with my old PC path C:/Users/Mike/My Music/Music. I went to the "Select and Replace" feature of Word Pad and put the old path to be replace with the new path, C:/Users/namas/Music. It took about 10min for it to run (my library file is almost 10MG). When it completed, I saved the file, started up iTunes, and all my playlists showed up!