Music appears and disappears all the time from the iTunes Store due to artists and record labels deciding they're not making enough or for other reasons. If you backed it up, you still have it. If you didn't, it might have been withdrawn.
If it's not in your purchase history, though, I would give Apple a call and discuss it. Have your iTunes receipt email handy.
yes it can especially if apple changes the album or decides to remove the album for copyright claims you might have to call apple customer support for this one because they can look it up on there end and fix it
iTunes has (for as long as I can remember - although, I may be going senile) never shown "Purchased" rather than a buy link on my purchased content when I go back to look at an individual item in the store at a later time.
To see your purchased content, when you initially open the iTunes store, you need to look at the list on the right side of the store and click on the "Purchased" link there. You may need to sort by "All" to find the one you're looking for.
There is, also, no place to view "all" your purchased content. You have to go to each individual iTunes store; Music, Movies, TV shows, Books, Apps... and click on that "Purchased" link at the right side of the page and can then view all your purchased content related to each area of the store.
@OP Are you saying the files are missing from your music folder? If the icons for those albums are still in your library delete them before following the advice above. As long as the icons/artwork/library entries are there the store thinks the files are there even if they aren't actually present.
When I go to my music library , the albums have completely disappeared. No icons or anything. They are not in my purchased section in itunes anywhere. When I look for the album in the store it shows the full asking price in green rather than the grey purchased. It's as if I never bought them !!