My screen goes black??

What really worries me is that this iPhone is new. Well my previous iPhone got water damaged and it's warranty hasn't expired so I used it. I paid 300€ and they gave me a new one. now after about three weeks of owning my new device the screen goes black suddenly, but the phone still works, I get messages, calls notifications and I can lock/unlock it while the screen is dead. The home/lock button restart solves this for a while but then the problem will appear again. Also after I try this the apple logo does not appear so that is a bad sign... I was wondering if I could sent it back to warranty (they told me the new phone has 2 New Years of warranty) and if they could fix it free of charge? Do you think it will be free? because I didn't drop the phone and it isn't water damaged cause I checked the little indicator. I believe they gave me a flawed phone. Whether it happened accidentally or all the phone that come from warranty are flawed I don't know but I want this fixed and I don't want to spend money again...