back light of my phone is not working.

Anup Adhikari, there are a couple of possibilities here. First, of course, make sure that your backlight setting are properly set. Tap the Settings icon on the home page of the iPhone, then the "Brightness" tab. Here you should get the slider for the brightness. Moving it to the right will increase the brightness of your screen. The other possibility is that the backlight in your LCD is no longer working, you can check that by replacing the LCD (may be you can borrow someone's phone to check it out) The next possibility is a bad coil on the logic board. It is a 6R8 and not an easy fix unless you have done repairs like that before. You can check on on this video and see if you are up to a repair like this. Hope this helps, good luck.