iPhone 5c screen not working

So my girlfriend bought herself a second hand iPhone 5c, for the past few months is been working fine untill last week When she dropped it, it had a few minor cracks in the screen but didn't affect the use of the phone, a few days after the screen started showing lines of green and red around the cracks until the screen just stopped working, I ordered her a new screen and it came this morning, when I fitted it, it was all working, a few hours later the screen has just gone completely black with a few green lines around it, just wanting to what could be wrong with the phone.