HELP PLEASE! My screen is black after screen repair

I have an iPhone 6 i had a screen replacement done about two weeks ago because my screen was cracked and it had colored vertical lines on the side and black dots. After the screen replacement my screen is black i tried holding down the sleep/wake button and the home button but nothing work. i opened the phone to make sure the cables wasn't loose. I was thinking that it was the screen so i had someone look at it and it wasn't that he tried a new battery and that didn't work. When i hold the volume, home,sleep button at the same time my phone vibrates it's not completely dead it still responding but the screen is black.. When i plug my phone into my laptop iTunes pops up and tells me my battery percentage and backup information and all that information. It's been TWO WEEKS and i want my phone back. I have no idea what to do i don't want to get rid of my phone because i didn't back up any of my information. PLEASE HELP!!!!!!!!!