Sony Ericsson :: Vivaz Screen Blacking Out

I don't know whether this problem is with the phone itself or whether this is a common problem, the screen goes black when left for a minute and when i try to unlock it i can't do anything at all, it doesnt even come up with the option to unlock the screen it just stays blank.I can receive calls when its black by pressing the green call button but i cannot look who it is or who is texting me. i have only had this phone for 4 days.