I bought a song on itunes using my ipod touch but after syncing it to my computer is doesn't show up on either libraries (one on ipod and other on computer), is there a way for me to retrieve this song because it shows I paid for it.
Did it show up on your Ipod after you downloaded it? I recently experienced the same problem with my Ipod showing the download process but no song when it was complete. Itunes will send you a receipt to your email address within a few days of purchase. The email will contain a link "report a problem". Just follow the steps in that link and they will email you another "download" link at no charge.
try clicking on the itunes store, then going to where it says 'account' on the right hand side. this gives you various details, including a purchase history.
by each order date, there is a little arrow, if you click that, there is an option to report a problem. drop apple a line, they'll almost certainly email back pretty quickly telling you that the song will be available to download next time you load itunes.