Your iPhone 6 plus is less than 1 year old. As long as you did not drop it to cause damage to the camera, it is still under Apple's warranty. Take it to an Apple Store if you have one near by and they will make you whole. If you do not, then call Apple support. Do the store though if you can, you will most likely walk out with a phone that is working.
There is a known issue about this. Mine just stopped focusing as well. I found an article about this where you can enter your serial number to find out if your phone is one affected. They will replace the camera at an Apple Store.
Check your serial number here: