My iphone 4 music won't sync

My iphone 4 had music on it but now itunes won't let me syc my music to my phone! What's wrong with it?

ITunes 10.5 or later is needed to support iOS 5, iTunes 10.7 or later is needed to support iOS 6. Check it first.

You can go to Cnet to find a freeware "Syncios" to help you transfer music from PC to iPhone.