Why has my music disappeared and how can I get it back?

Question

So its been a few weeks since I last used my iPod Touch (3rd gen, 64gb), and this evening I decided to resync it with iTunes as I had added a couple of new albums these last few weeks (Coldplay and an xmas album, not that it matters of course). After syncing, my iPod now only shows these two albums, but not the other 50gb+ that I have on there. They're still showing and playing in iTunes, and everything checks out when the iPod is connected.... it shows I have it filled with the right amount of audio/apps/etc. But no matter how many times I resync, the old music just isn't showing up on my iPod anymore. I've synced plenty of times before without having any problems.
Anyone have any advice? I'd prefer to fix it without wiping if possible, as 55gb of music and apps takes a **** of a long time to restore.

Answer 1

I am not sure if my problem is identical to yours but will post what I have discovered for the missing Greenday music from my daughter's ipod. It has been an aggravation to have to keep putting this music in and after a period of time, it disappears. This is what I have discovered as of yesterday. I hope it helps someone.
How to keep music/picture files in your ipod:
I have noticed that my daughter's music and pictures that have been put in her ipod from our PC files happen to disappear at random times. The photos have disappeared twice and the music 4 times. I think  I have finally found a solution to this problem.
When you are in itunes, you will notice that music files have checkmarks beside each song. The downloaded music files (bought from itunes) are the ones that always stay in her ipod. It is the PC transferred music that disappears. This holds true for the photos too. The ones she takes with her ipod don't disappear but the ones we put in her ipod from our PC do.
When in itunes, you will see the checkmarks beside each individual song when under music category. That is the itunes menu and the songs that have checkmarks that are your PC files, do not automatically sync each time that you sync your ipod. The downloaded stuff from itunes automatically does, by default, so I will explain what needs to be done so that your ipod knows this too.
Connect your ipod into your PC. Go into itunes. Click on "ipod" (under "Devices" in left hand column) to whatever you named your ipod. We call my daughter's "marley". So click on "ipod name". On this page you will see your ipod name, capacity version, software version, serial #, etc. If you want your music, the categories are listed at the top of this page, so click on music. You will see, at the top, a checkmark beside "sync music" and boxes underneath with "selected playlist, etc." and other boxes with "include....etc.". Put checkmarks in boxes for which you want. At the bottom right hand corner, you will see a box called "Albums". In this box, anything that you have downloaded from itunes  is checkmarked by default but any files that you have put in from your PC will be unchecked. Put checkmarks in these boxes, click "apply", and sync your ipod after doing this. You shouldn't lose these files again.
The same thing applies to photos. Go back to main ipod page mentioned above. Click on photos. There should be a checkmark in "sync photos from my pictures". Also the circle with selected photos should be filled in. Underneath that you will see "Folders". Put checkmarks in the folders that you want. This should help you to not lose photos. I have not had issues with video, etc. as we have not put these type of files into ipod yet. This has only applied to music and photos as of this post.
Hope this makes sence and do hope that it helps someone.