My iPhone 6 has shut down and has a black screen, can't reboot either, help!

See: What to do if your iPhone or iPad will not turn on

Thank you so much Lawrence- Your solution worked perfectly and my iPhone6s is open. 🙂 You saved me from having to set an appointment with the tech gurus.

You're welcome. This falls into the age-old maxim, "when in doubt, reboot."

I guess I would have to take my iPhone 4s to the store then?, Since my power/sleep button is broken.

If either button is broken the only way to reboot is to wait until the phone shuts off with a low battery. Which won't happen if the screen is black. You can take it to an Apple store, but they will probably suggest you replace the phone. I think the cost for a 4S is $149, but I wouldn't invest that much in a 4 year old phone.