latest itunes update?
I'm sorry I'm not an expert on this techie stuff, but I too have been having problems since the last itunes update. Once you've done the tests already suggested, you might need to ponder whether or not it is itunes itself. My problem is that of the hundreds/thousands of tracks on my itunes, (not me - daughters!!), about 1/3 of them have stopped playing since the last update.
I can only guess that the most recent itunes has done something "clever" with its security.
All the tracks that I know I've put on there clearly work, but some from elsewhere in our modest domestic network, and some that were loaded to this pc, won't work.
I've used it as a reason to tidy up the tracks deleting those that won't work - boring in the extreme, but worth doing in our case. I did go looking for the tracks to see where the pc had put them, but didn't get any joy from that.
The new itunes tracks don't copy over to the Nokia software so well, and that is deeply frustrating as I use my Nokia phone as my ipod.