itunes not sync with iphone

i had used itunes many times with my iphones (2 iphone 4 and 1 iphone 3g) ,from last  2-3 days my apps and music is unable to sync and noy any error there, .just sync completed . i m bit confused then i changed usb data cable , i thought may b here's problem , but after all efforts same problem . apps and music not sync . suggest me what to do ..., and one more prob it shows no service