Samsung Galaxy S3 not turning on My Samsung Galaxy S3 battery died. A green robot with download message appeared when I plug it into a wall outlet to be charged. Seeing this for the first time, I thought it might have been some virus, so I removed and replaced the battery. The same thing happened again. After removing and replacing the battery a second time, the robot disappeared and my phone has been dead since, despite being left to charge for 5+ hours and several attempts removing/replacing battery and using the 2/3keys turning methods and plugging it directly to a laptop. How do i get it working. If fast boot is a possible solution, how is that done?
you can Follow this Guide to Flash your Galaxy S3's Firmware again. 

Things you have to do for this are as follows 


Download the latest firmware for your device at sammobile
Copy the firmware to your device either by Recovery mode or Samsung Kies or Samsung ODIN
Flash the firmware on to your device using ODIN


The Complete process with pictures is provided in that link. Hope it helps