Why can't I see what's on my screen?

My iPhone 6's screen is black, but it vibrates when I plug it in. Why can't I see what's on my screen? And how can I fix it?

I was using Snapchat when I got a text and my phone froze. Naturally, I just tried locking the screen and pushing the home button. Apparently, it made my screen go dark and now I can't see what I'm doing. However, I know it is still on because it vibrates when I plug it in to charge it.