I cannot sync or back up iphone. it says phone has never been back up which isnt true. When i try loading music or back it up it says syncing for 1/4 seconds and then does nothing. very frustrating, what do i do?

I cannot sync or back up my iphone to my itunes through my mac. It says that this phone has never been backed up to this computer. This is not true. When i try uploading music or back up the phone it says its syncing for 1/4 of a second at the top and then it disappears. this is very frustrating and im not sure how to fix it. Ive restarted my phone and computer and still nothing. It also says on the bottom banner that i currently have 49 songs on the phone and i dont have any. how do i fix this?