Why does my screen not work after water damage replacement?

I have the iPhone 4, obviously. I dropped it in the bath a couple of months ago, instantly took it out, dried it, it shut itself off and I put it in rice for a couple of days. I noticed that the phone kept turning on by itself while in the rice because voice control kept activating. The screen, sound, touch, the home and lock button worked fine. After a few days, the phone died completely. I left it in rice for a few more days, then plugged it in to charge. The screen was dead, no touch response, no backlight or image. I pressed the home and lock button to see if I could get a flash from the screenshot, I only heard the sound. That means that both the buttons, the battery and at least a little of the phone still functioned. Looking at the image in my computer, the screenshot function still worked. Today, I replaced the screen finally. I ordered an iPhone 4 screen online, and followed the instruction on iFixit, replacing the screen with ease. Unfortunately, the results were the same as previous. The screenshot still works, just no response from the new screen. Of course, I'm an incompetent 15 year old girl with no experience dealing with the inner workings of Apple products. I've been told to soak the board in alcohol, but I was wondering if there's any other way to solve this or at least know what the problem is and what I can replace, etc.