I have an iPod Classic 80gb. It has about 32gb of songs and videos but don't show/play them. I connected my iPod on my friends system and since then it shows 32gb used but doesn't shows my mp3 files on iPod but when connected to a computer it shows all the files?
How can I make my iPod to shows and plays the files it has?

When connected to the computer, remove the files from your iPod on to you computer. Open iTunes. Import all these songs in iTunes. Reconnect your iPod and follow the onscreen instructions.