What country are you in, and what happens when you try ? Music can't currently be re-downloaded in all countries - if you are in a country where you can re-download music then it should appear under the Purchased link under Quick Links on the right-hand side of the iTunes store home page on your computer's iTunes : re-downloading.
If under that link your music shows but it doesn't have the cloud symbol for re-downloading then do you still have the 'pointers' to those tracks in the Music part of your iTunes library, even though the actual music files might not be on your computer ? If you do have the 'pointers' then iTunes assumes that you still have them in your library and that there is no need to re-download them - in which case try deleting one of them from your library and you should then be able to re-download it. If that works then do the same for the others.
If music doesn't show as an option under the Purchased link then I assume that you aren't in a country where music can be re-downloaded - do you not have them on a backup ? If not then you can try contacting iTunes support and see if they will grant you a re-download : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page