Fixing my itunes files

I have quite a bit of music about 40 gig. It was too big for the regular drive so i copied the music to an external drive. I recently lost my ipod classic so I had to go and look at my itune file on my laptop. I started it and everything looked great. I had my playlist which is the most important but when i tried to play a few of the songs, itunes could not locate the music. I looked and did not find the music files so I copied them from the external drive. Funny thing is, this took place over a couple of days and itunes recognizes some of the music but not all of it. Question is what can I do other than importing the music again, to get i tunes to recognize the music. It has already changed a few of my playlists files to the musics it recognizes. Since then, I have purchased a smaller shuffle, so my playlist are important.