How to fix a dark screen

Amit, rice is great for Gallo pinto :-), but it will not fix your phone. Rice does not prevent corrosion, it is the equivalent of doing nothing. The very first thing to do is to disassemble your phone. Use this guide. Once you have it disassembled clean it with +90% isopropyl alcohol. Do not substitute this with other cleaners :-) Use this guide to see how to clean the phone. It was written for a 3G but all the steps are pertinent to your phone as well. Check for any missing or charred looking components. Once you have it cleaned, replace the battery and reassemble your phone. Reevaluate for any damage.There is really no a surefire way to say which part is the one that gives you the problem. I would recommend that you start it systematically. Make sure that your replacement screen really works. If it does, take a close look at the backlight IC and coil on the logic board. That repair is a bit more difficult due to the fact that those are SMD components. Water damage is the hardest to diagnose and to repair since you never know exactly which part has failed. Even if you do everything right, there is still a good chance that your phone will never be the same. Hope this helps, good luck.