I just downloaded the iOS 7 on my white iPhone 4S?

Then I was fixing up my contacts and adding a picture to a contact and all of a sudden my screen just turned black! It won't turn off or go to the home page it's just there. The black lighted up screen. I don't know what happened! Please tell me what's going on or how I can fix this! I would really appreciate it. I'm using my iPod right now for all of you courious people out there.

Hold down the home button and the power button at the same time. Keep them both held down past the power off screen and ONLY release them when the apple logo appears. That should sort it.