How do I unlock a black screen?

My phone went in the water. I turned it off right away but every time I turned it off it would come right back on, on its own. Two hours later I got it in the rice. . Then the screen went black but Siri was still yelling at me about being "unavailable" from within her bag of rice. Now 48 hours later the screen remains black but when I plugged it into the computer it launches iPhoto and I was able to download my photos but it won't let me backup because my screen is locked. I can't unlock the screen because I see nothing. It will communicate with my laptop and I can answer a call but I would love to at least back it up before I take other measures. Any ideas how to unlock a dead screen?