Raz, I'm sorry, but I can't help, I don't know about iTunes. What I CAN do, however, is rage about how shitty a player it is (Winamp for the win, iTunes can't even get a REAL music library together and...and...) and how hopeless and arrogant Apple customer service is. Grrr!
Least helpful or constructive post ever. Moving on.
Certainly not a particularly common thing, there was an issue with iTunes libraries about 18months back after one update on windows where iTunes changed from calling it iTunes media to iTunes music. So my first suggestion which I'm sure you have already done it to have a good look to see if it isn't the case that the items are sitting in a slightly different folder and the library itself has simply dropped the link. Issue you get with this sort of thing is that 99.9% of cases are pebkac, normally the music is in there somewhere. Is your iTunes library consolidated, see cases pretty much daily where peoples iTunes libraries are a collection of music from all over the place and it causes all sorts of issues.
Secondly assuming the music is defo gone from the system, bung a mail to the store support guys. Explain as you have done, if you can give them the invoice numbers makes life much easier. You can get those by going, store, my account, view account, purchase history. Store chaps are normally pretty sound and reasonable. Keep it concise and bung the store invoices in and they will like it even more
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Thirdly you may or may not consider it worth it for 300 odd tracks but you can often get stuff like this back using basic data recovery software.
Also not that it helps much at this point but 2 things to consider for the future. Firstly specifically relating to iTunes,
iTunes match is an excellent way to secure an iTunes library. It also matches all music regardless (to a degree) of imported quality or import origin. If you want high quality replacements for music that fell into your computer this is one way to do it. Secondly, it's always worth having a back up. The cost of a hdd to back stuff up isn't that much and it makes it much easier to avoid this kind of annoyance. Not used any back up software in a while for windows, no doubt someone can name one, or even a quick googling will let you know.
