Why can't I use screen on IPhone 4s?

My phone was working fine this morning. Had USB connected to phone and car to play music. Now I can't touch the screen or open apps or slide to unlock the phone. But if I tell siri to open my messages then I can type perfectly fine. I can't even turn my phone off to restart it. How do I fix this?

Will it plug into iTunes? Try doing a full restore and see what happens