Sincing I-Pod Touch after downloading latest latest version 4.0 software.

When in Itunes a notice came on screen to update the latest version software for my I-Pod (Ver 4.0) this I did, now my I pod will only sinc part way then comes up on screen "Cannot sinc I-Pod there is a missing disc" I do not understand what this means, I cannot redownload the previous version of software for my I-Pod. When I try to re-update the new version (Ver 4.0) it just states that I already have it. Also on the I-Tunes page when I-Pod is plugged in at the bottom where it shows the graph on what is on the I-Pod, *MUSIC and VIDEOS* is not shown but apps and other are shown. I just dont know what to do, I dont want to lose anything off my I-Pod. Please Please can anyone help me.