Songs that I have purchased are missing in iTunes (file not found/exclmation point), and do not show up in the iTunes Store for download.

Question

I have a laptop that has (or had) pretty much all of my music, both burned from CDs and downloaded from iTunes. My 17 year old son on occasion syncs his iPhone and iPad with this laptop. As of the last sync (as best I can determine), many songs that I purchased in the past few years are now missing, in that they have an exclamation point to the left of the song title, and when I try to play the song, I get an error message that says "The song could not be used because the original file could not be found" or something to that effect. As it turns out, the file seems to be erased. This has happened multiple times, and I could be wrong, but I believe that there are some songs that had the exclmation point last time he synced that now don't, and vice-versa. There is not an obvious correlation or grouping as to why these songs are missing (e.g,. they were not all purchased on the same date, not all songs from one artist are missing, etc.).
Since most of these songs have been purchased through iTunes through my account, my reaction was to go to the iTunes store, log in with my account, and re-download them. Problem is, all of these songs instead ask me to purchase the song, instead of displaying a "cloud" logo. Therefore, it appears that I have to re-purchase these songs, which does not sound correct.
Questions: Does anyone know, most importantly (to get an immediate fix):
1. How can I re-download these songs and why doesn't iTunes Store recognize that I have already purchased them?
and perhaps as important (more of a long-term question):
2. Why does this happen and how can I prevent this from happening again (since it has happened multiple times)?
Thanks.

Answer 1

1. iTunes won't offer cloud downloads for songs that it "thinks" are in your library, even if it "knows" the files are missing. If you've exhaustively searched for the missing files and there is no prospect of repair by restoring to them to their original locations, or connecting to new ones, then delete that tracks that display both the missing exclamation mark and are of media kind Purchased/Protected AAC audio file. Don't hide from iTunes in the cloud when asked, close iTunes, then reopen. You can download from the cloud links or iTunes Store > Quicklinks > Purchased > Music > Not on this computer > All songs > Download all.
2. Why? Not sure, perhaps 3rd party tools or accidental key presses combined with previously hidden warning messages when trying to organize the library. There is a hint that using the feature to downsample media as it is synced to a device may also be involved, though I've not replicated it. Whatever the reason a backup would protect your media.
tt2