Why is my phone is completely unresponsive

First of all, my samsung s4 is barely 2 months old but has been dropped and the screen cracked but it was repaired. The guy who repaired it said there was nothing else wrong with the phone.

Today I was in the middle of using it the screen froze up and the phone wouldn't shut off after i pressed the power button the first time. The second time I pressed it there were white horizontal lines on my phone. So I took out the battery and put it back in and the phone wouldn't turn on. I knew my battery wasn't anywhere close to dying but I plugged it in anyway and there was still nothing not even the red LED.

I thought at first it might be the battery so I just exchanged the battery for the fully charged one i keep as a back up and still nothing even after i plugged it in. I've tried the 3 button method and that didn't work. HELP!!!!