Iphone 5C turn on problem!!

Hello! I have a problem with an iPhone 5C. The phone charges, but it doesn't turn on. I can see the USB-cable showing me that I need to charge the phone, then the red-battery after I get it to charge. I tried to force the turning-on by pressing the lock button + home button and nothing happened. I took it to service and I had it turned on there. The assistants said it has an electric problem, but I guess they don't know which is the problem either. Still, the phone doesn't turn on. The buttons are working, but the phone doesn't show any signs of turning on. What can I do about it? Which is the problem it has? Thank you. I have iOS 9.3.3.