library issues with iTunes 12

I have just updated iTunes to version 12.3.3.17 and now NONE of my files (almost 3TB) are present.

Having changed the location of my library to my external drive (where I keep it) everything now appears not to be available or recognised... instead only the files I purchased on iTunes (rather than imported) appear to be available and only if I re-download them.

Quite frankly I am NOT impressed - does anyone have any ideas?

Thanks