Why does my iPhone 4S go blank due to over heating? My iPhone 4S is just 2 weeks old and since the last week it enters this weird mode where the screen goes blank and I can't access the phone, but if someone tries to call me, it rings, proving that the phone isn't turned off.

The store guy said that it's happening because Im using the phone too much which is causing the phone to overheat and go on a sleep mode.

My phone goes on this wierd sleep mode atleast twice in a day! Even during the night!  What can I do?
The store guy is telling you crap, there is no such thing as a sleep mode that would exhibit the symptoms you describe.  Demand an exchange.