This “No light on display” is usually an internal issue with the LCD connector. However, Apple recommends some processes to practice on your phone to make sure the black screen is not a software glitch.

• Force Restart your iPhone.
• Connect the lightning cable, charge it for 10 minutes. Then try force restarting the iPhone.
• UNCONVENTIONAL: But press the screen into the phone so it may reconnect any dislodged LCD connector. This fix is not recommended however it resolved a lot of the user's Black Display issue.

If none of the fixes work to resolve the issue on your iPhone, you may need to get it looked at by experienced professionals who specialize in iPhone issues. Here at ifixit bd, we have dedicated and experienced technicians who specialize in resolving No light on display issues on your iPhone 6s. We are a service center that operates based on 4 different and convenient points around Dhaka, Bangladesh.