I used my IPhone for only one week and a half. Today, after I received a call from a friend, the screen turned black no matter what key I pressed (including power off). I used another phone to call my IPhone, and I can hear the ring, but the screen remained blackout. After around 5 minutes, it returned to normal. Very frustrated. Any suggestions? Shall I go to Apple store tomorrow to swap a new one?

First thing to try is to reset the iphone

press power off (top button) and home button at the same time and wait ( past the turn off message ) until it restarts with the apple logo.

If that fails, next thing to try is to restore the iphone via iTunes.

One or other should fix it.

Sounds like the phone just got confused...