It appears that you're unable to use the rear camera on your iPhone because you're receiving a message that "Flash is Disabled" and the phone needs to cool down. I'm happy to help.
Your flash isn't working
If you take extended video with the flash on, or you’re using the device in a very warm environment, you might see a message that says the flash is disabled. The message remains until your device cools down.
Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures -- Review the information in this article to learn how to keep your iPhone within an acceptable operating temperature.