iTunes won't finish syncing.

For the past few days since I updated the software on my mac, my itunes hasn't enabled my iphone to finish syncing.

This has now meant that the music I once had on my phone is now coming up grey with a circle next to it meaning it's not on the device anymore.

However the music that was downloaded previously from itunes is still on my device.

Can anyone give me advice in how to get my music back and to somehow get my itunes to finish syncing, it gets through all the stages and then just remains on the last one for hours before saying it can't sync it.

Help please! x