The first thing to do is see whether the missing songs are on your computer. Use Windows File Explorer to look for any one of the missing songs. The filename of a song is usually the song title (or at least, the filename contains all or part of the song title).
If you do find the file, go back into iTunes and use the "Would you like to locate it it?" (You have to locate it, not iTunes). Then navigate to the correct location. If you successfully locate one track off an album, then iTunes should be able to locate the remainder of that album (it will ask you if you wish to use that location {the same folder} to look for other missing files).
If the songs are not on your new computer, then it sounds as though your copy was unsuccessful. Try again.
If you're still having issues, try to work out what sort of music is missing:
- Amazon files may be in a different folder (one named Amazon MP3)
- purchases from the iTunes Store require that your current iTunes is signed into the same Apple account that you used to buy the songs originally
- CD rips may be in different parent folders as Apple changed the location from iTunes Music folder to the iTunes Media folder. You may also find file for CD rips in My Music\iTunes\Previous iTunes Libraries\iTunes Media\Music. Why? Who knows? All I know is that over the years, my iTunes programme has used all three locations
iTunes is like all other software, is has its quirks and on occasion, it is frustrating (I know).