how do I fix albums which are fragmented

I am using iTunes ver 12.3.044. When I imported my music library from an external hard drive back onto my laptop (after a reformat and reload) a lot of the albums are fragmented. Also a lot of albums which I am sure have artwork come up as "unknown album" or it says there is no artwork. I am trying to figure out how to 1. consolidate the albums so they have all the correct songs, and 2. how to get the correct album artwork? Has anybody else run into this or did I do something wrong?