Is the LCD Broken? If not, what is?

I dropped my Iphone the other day, and right afterwards the screen worked fine lighting up and showing some response to touch, but I could not get it unlocked (I managed to power it off after multiple tries, but I could never slide the unlock bar over). I figured the screen, which was cracked now was the problem, luckily I already had an extra screen so I replaced it myself (something I've done before). Now the iphone turns on, and you can see the screen but there is no back light, and no response from the screen at all. What is wrong exactly? Is the LCD broke, or is it the screen/digitizer? Thanks!