Exclamation HTC Sensation XL - Black screen issue

Hello, I bought an HTC Sensation XL with broken screen. I change the screen but the phone doest not show nothing. When I press the power button it vibrates once and then nothing. When I plug it into charger the led is green which means that the battery is full. I tried to hold down volume down button and power button but nothing happened. Can you give me some ideas what to do for my phone. Thank you in advance.