My i phone 4s won't work

What happens when you try to slide to unlock? What do you see on your screen?

When you press and hold the power button what happens? What do you see on the screen?

Did you try and reset your phone? Press and hold both the Home button and the Sleep/Wake button continuously until the Apple logo appears. Then release the button and let the device restart. You will not lose data doing this. It's like a computer reboot.

Do what Ralph9430 has suggested. Most of the time, rebooting fixes many software issues. Just hold the Home button and the sleep button for a few seconds until the phone shuts off and the Apple logo appears then let go.