Where is your backup copy? This is one of the many reasons a backup copy of all important info/docs/music/pics/etc is always basic.
If for some reason you have failed to do so ( not good), then you can transfer itunes purchases from an ipod: File>Transfer Purchases
From the terms of use of the itunes store ( same policy as Amazon and Walmart online music store):
"PRODUCTS CAN BE DOWNLOADED ONLY ONCE; AFTER BEING DOWNLOADED, THEY CANNOT BE REPLACED IF LOST FOR ANY REASON. YOU SHALL BE RESPONSIBLE FOR BACKING UP YOUR OWN SYSTEM, INCLUDING ANY PRODUCTS PURCHASED OR RENTED FROM THE ITUNES STORE THAT ARE STORED IN YOUR SYSTEM."
http://www.apple.com/legal/itunes/us/terms.html#SERVICE
You can try e-mailing itunes support to ask for an exception, but they are not obligated to do so, particularly if you have failed to maintain a backup.
As this is asked and answered several times a day, you can try a forum search. Zevoneer explains the options very well.
There are third party programs ( not supported by Apple) that you could try at your risk as well.
