Tap on the home button twice, in order to see apps you have been using lately. Find a camera app, and swipe up to close it.
If you have been using many apps lately, most of them are still running in the background, which may cause this strange issue. To turn them off, repeat the same steps used for the solution No.1, and instead of closing only the camera app, close other apps as well.
Press and hold the sleep/wake button, and when the slider shows up, drag it, and turn off your phone. Repeat those steps, and turn your phone on.
For some users, the new iOS version was the solution they have been looking for. Upgrading your device can solve some other bugs along the way.
Backup your files first. Click on Settings, General, Reset, and choose Erase all Content and Settings. In order to confirm the action, press OK. When erasing part is over, set up your iPhone 6 as a new device.