not so good

Assuming you have already tried to hold the sleep/wake (power) button, follow this guide

If your iPhone, iPad, or iPod touch doesn't respond or doesn't turn on - Apple Support

I just had the same thing happen to me. My iPhone 6 went dark and wouldn't turn on, so I started looking in this support site. I followed the steps that Jay-Ray linked to, and thankfully my phone started up with the first step. Yay! 🙂

Glad to see it worked!