iphone not communicating with computer

iPhone4 not connecting to computer - when i first accessed it it recognised it and made it so i could start using my phone but now after plugging it in again nothing is happening - it doesnt even charge!! however if i turn the phone off and plug it in then it turns itself back on again but again it does not charge or connect to itunes - i have tried services.msc in the run menu and the apple device has started so i am very confused - please help only got my phone 2 days ago!!!xxx