Can't move songs from iTunes to iPhone? NO NEW LAPTOP OR PHONE?

I haven't got a new laptop or a new phone or updated anything. Yesterday I opened my iTunes to find all my music gone, it was still in my files so I was able to copy it across again. Since then I have been unable to copy any music over to my phone. When I press 'sync' it just doesn't add anything new, and if I try to drag items across individually I just get a little red circle with a line through it.

Don't really understand what's happening as I haven't had any updates or anything. I literally haven't changed a thing, it's just started doing this for no apparent reason. As I'm sure you can understand, this is one of the worst things that can happen to a person. I would be infinitely in debt to anybody who can solve my problem.

Type in on YouTube it will show you videos on how to control iTunes.