Iphone 4s won't connect to itunes?

I just had it connected to itunes 2 days ago and then I went to put more music on my phone and it won't connect, I don't understand.someone help me?

Added (1). I restarted the computer and unplugged the usb and is still not working -.

You might have to restart the computer, or try unplugging the USB and then plugging it back in.