My iPhone 4s won't sync to my iTunes anymore for some reason. It's been over a week since it has synced, what am I doing wrong? How do I correct this?

My iPhone 4s won't sync to my iTunes anymore for some reason. It's been over a week since it has synced, what am I doing wrong? How do I correct this? I hooked up my iPhone to my computer via USB, and it charges but doesn't sync to my iTunes. Not even an icon indicating my phone is connected appears in my iTunes. Please help. I restarted my computer, restarted my phone, the restrictions are off, my phone is up to date - I don't know what else to do.