Lost Many Music Files In iTunes Library

When I transferred my iTunes library to a new computer I lost a significant number of my music files. But my iPod touch (7th generation) has retained all of the library. I transferred the library from an old Dell Inspiron to a new Dell Inspiron 15 3520.

I have read something about a third party app that could transfer the missing files.

I would appreciate any help or advice you could give me.