CopyPod, iPod Updater, iTunes and hard disk space solves this problem
I had exactly the same problem with my 40G 3rd generation iPod and 33G in use. Everything was working fine, I've added an album to my iPod (iTunes with manual update), disconnected it from my windows PC, switched on the iPod: No artists, playlists, songs...
I did a reset by pressing the play and menu button. No use. My About window of the iPod said: 0 songs, capacity used: 33G. I connected the iPod to the PC again. Nothing in iTunes except an error. In Windows Explorer I was able to see al the MP3 files in the iPod_control\Music\F00 to F49 hidden folders of the iPod Drive. So I copied these to my hard drive (check first if you have enough space). Then loaded it into iTunes (add folder to music library) and that's it.
Two awkward things with that method: my playlists are not copied and all my song ratings are not copied. I deleted all the songs from the hard drive again and tried ephPod, but unfortunately it did not recognize the songs on the iPod (same as iTunes). Then I used a program called CopyPod and that solved my problem. CopyPod listed all the songs of the corrupted iPod, together with all the playlists AND all my song ratings and playcounts.
I copied all the songs to my hard drive (mp3 format if you choose Copy To iTunes, m4a format if you choose Copy to folder). The process takes an hour or so. I then ran the iPod Updater program (download the latest version from the apple site) to format the iPod and used iTunes (with all my saved songs copied by CopyPod) to put it all back to the iPod. Everything is working fine again!!
but it took me some time...
However, I don't know exactly what caused this problem, I guess a bug on the iPod while synching some songs. After copying all the songs to ITunes, CopyPod showed me an ErrorLog with 10 erronous songs which were listed on the iPod but not found. I guess that could be the origin of this problem.
CopyPod is not freeware, it costs 20 dollars but my 4000 song ratings and playlists are worth it in my opinion. Hope this helps.