why is sync and apply greyed out

Hey guys,

I have a 4s , with fully updated software. It was working great and then the other day when i was trying to put some more music on it, I wasnt able to. The apply and sync buttons are greyed out. I have completely uninstalled itunes, and reinstalled it again, restored my i phone and reset it. But to no avail. When I plug my phone into someone elses laptop, and act as if im going to sync it, the sync and apply buttons are greyed out. And when I plug someone elses iphone into my laptop the buttons are greyed out just like they are for when i plug my phone in. However when i plug my ipod in they buttons are fine and i can easily put music on it.

Ive tried everything I can think of. Please if you know how I can fix this, please let me know. Would really appreciate it, im going nuts.

Jack