iPhone 6 won't charge

I just plugged my iPhone 6 into my car charger, which has worked fine for the last year and now the software is coming up with a warning saying that "This product may not be supported". I also tried plugging it into the power socket at home with all genuine apple cables and power supply and it shows the same warning. I also tried plugging it into my iMac with a genuine apple cable and it came up with the same warning. Now my phone has lost all power and I have no idea why it just started coming up with this warning all of a sudden. I also tried the hard software (Home and on/off button) reset that you suggested but since the phone won't charge and has no battery life left and as such now it won't work.

Now that the iphone's battery is flat, it comes up with the battery symbol in the middle of the screen and the cable warning symbol. Once I plug in the cable the cable warning symbol disappears which would indicate that the software has recognized that the cable has been inserted. However the iPhone will not charge at all.

Any idea on how to fix it?.