For tracks purchased from the iTunes Store, delete the listings from iTunes, go to the iTunes Store, log into your account, and click the Purchases link under the Quick Links. From there you should be able to re-download some or all of your purchased content. Note that not all content has been licensed for re-downloading in all countries at this time. You can see what content you can download here:
For full instructions, see:
iTunes Match might work for other tracks - I believe it looks at the listing in the library, not for the actual track files - but I'm not certain. Perhaps someone else can help. iTunes Match would only provide tracks at 256kbps which may be lower than what you had previously. For tracks not purchased from the iTunes Store, then assuming you don't have a backup the best thing will probably be to re-rip them from the original CDs or get them from the original place you obtained them.