Ipod won't mount even after back from apple repair

I have a 5 gen ipod running on a G5, all updated with latest OS and itunes, and last week having been mounting the ipod all day, it decided it wouldn't recognise it. Cannot mount on anyones computer, and now I have it so it launches itunes and says it won't recognise it. Sent to apple for repair and have received it back saying they can't find anything wrong with it, but when I plug it in, itunes still can't recognise it. Anyone any ideas? I have restored the ipod, updated my computer etc but its not working on other peoples computers either.

Many Thanks in advance.