Thanks for reaching out to the Apple Support Community. I understand you have had some trouble with the flash in your iPhone 6s, and I have some information that may help. Have a look at the article below and follow the steps in the section titled "You see a closed lens or black screen".
Get help with the camera on your iPhone, iPad, or iPod touch - Apple Support
If the behavior persists or you run into any trouble, we're here to help.
Best Regards.