My bone broth dehydrated in slow cooker overnight
The temperature will have been high enough to be safe, so don't worry.

What you'll get is a darker, quite tasty stock. It's not suitable for everything (it's likely to be cloudy as well as dark) but can be very good. When I first got my new slow cooker it tended to do this, because it runs a little hotter than my old one. Running it on low instead of high (starting with boiling liquid) solved that.

You can also use the concentrated stock you've made, just remember to use much less than you would of normal stock.