My Samsung Galaxy A5 screen is blank.

My Samsung Galaxy A5 screen is blank.

i was just making a call on whatsapp and my Samsung Galaxy A5 screen turn blank, i cannot see any thing on my display, but my phone is working if someone send me a message on my Samsung Galaxy A5 i can hear the message tone or ringtone but i cannot see any thing.. please guide me what should i do, i just bought it 1 month back.