IPhone 5s won't switch on, but when I can hear Siri speaking when I press the home button?

My dad was on Facebook, and it wouldn't load. Like, just blue borders etc. So he closed all the apps & tried re-opening Facebook and it still wouldn't show anything except blue borders. So he restarted his phone and there were no visuals on the screen. But when he held the home button, we could hear Siri's voice. Any idea why this might be happening? Or suggestions on what he could do?

You can try resetting it by holding the home and power buttons simultaneously for about 10 seconds. If it resets correctly, you'll see the apple logo and it'll reboot.