My ipod can no longer sync to my PC?

Question

since the 9.1 upgrade, I had the problem of ALL my songs being deleted from my iTunes, I got them back off of my back up hard drive, but now when I plug in my ipod classic, it tells me that my ipod is synced to another pc, now I have never plugged my ipod into another pc, so how could this be?
help, I would like to be able to add and remove songs from my ipod, but it seems all I can now do is charge it?
any suggestions, I have already uninstalled and reinstalled itunes and reset my ipod.

Answer 1

*all the songs are there, but there are shown in light grey and not able to be high lighted.*
*I'm also not able to play from my ipod list, only from my itunes??*
Grey text when you look at the content and a lock symbol at the bottom of the screen just indicates that the iPod is set to sync automatically with iTunes. If you want to play the iPod songs through iTunes, delete songs from the iPod, drag songs from iTunes, add playlists to the iPod etc. you need to change the update setting to "manually manage songs and videos" as per the link above. The text will change from grey to black and everything will be directly accessible through iTunes
*I have seen a friend plug his ipod into some one elses computer and the same thing happened, it showed all the content but he was not able to access it, or modify it.*
Again that's most likely because the iPod was set to auto-sync with his home library. If you want to connect and use an iPod on more than one computer or with more than one library you need to change the update preference in the iPod Summary tab to "Manually manage music and videos" and click Apply. The content of iTunes and the iPod are not syncronised in this mode so the two can be different. You can directly access the content of the iPod and play it through iTunes and you can drag and drop whatever you want to the iPod from either library:
Using iPod with Multiple computers
*Before I backed up my itunes libary from my external hard drive, and itunes was empty, it would not allow itunes to be copyied from my ipod?*
iTunes will only give you the option to copy your iTunes Store purchases directly from an iPod to the computer, you'll find details in this article: Copying iTunes Store purchases from your iPod or iPhone to a computer
For everything else (music from CDs, other downloads etc) there are third party utilities that you can use to retrieve the music files and playlists from your iPod.