First you need to authorise you computer to manage your devices content as described here: http://support.apple.com/kb/HT1420
You can then transfer any songs purchased from the iTunes store as detailed here: http://support.apple.com/kb/HT1848
Any songs not purchased from the store can only be transferred using third party software such as:
disk aid: http://www.digidna.net/products/diskaid
Hope this helps
I had the same problem today when I reset my computer system. I found a program called SharePod that seemed to have helped. And it was free. . U can find it here on the CNET site: http://download.cnet.com/SharePod/3000-2141_4-10794489.html
Hope this isn't too late to be of help.