How to import files into iTunes library on PC

Yes I know this sounds like a real noob question but I can't figure it out.

I've had iTunes for years but when I recently got a new PC it didn't 'populate' the My Music list in the library. All the tunes I've bought over the years are in Windows Explorer in the Music folder in iTunes Media, but they have not been automatically added to iTunes library in the player. The only way I can do it is one track at a time either by Add File to Library or going into Windows Explorer and opening each track from there. The tracks are all organised into artist then by album in Windows Explorer. It is possible to occasionally open more than one if they are on the same album but not many.

Obviously, adding tracks one at a time will take ages as there are hundreds. Is there a quick way? iTunes used to do this automatically if I changed PCs.