trouble downloading purchased music on itunes on my laptop

i was trying to listen to some music from my laptop and when i go to click on a song a message appears where it says "the song (insert song title here) could not be used because the original file could not be found." the thing is though i know i didn't delete any music from my laptop but i checked in the file for iTunes and the songs that keep having this error pop up are missing. so i go to my account and try to re download the songs, they appear when i have it set to see all the songs I've purchased but when i try to click on the "not in my library" option there is nothing available to download. so basically i am unable to download songs/albums I've already purchased and i have no idea why. I've tried all the options given with the "how to" instructions but nothing works.