Chosen Solution
After changing cracked screen won’t light up. Plugging into computer gives 2 x “buzz” sounds and iphone is recognised on the PC but nothing on the screen. Completely dead in appearance
Reset, hold both Home and Power buttons until the iPhone restarts. If not fixed. Turn OFF iPhone, carefully open iPhone and reseat LCD Display Flex Cable Connector #1. If not fixed. Turn Off iPhone and try to reseat Connector #1 one more time. If not fixed. Reset, one more time. If not fixed. Restore, connect to iTunes and Restore the Firmware. If not fixed. Try new Display LCD or check your Front Screen on another 3GS.
If you have only changed the digitiser (screen) then the LCD (display) should still be working unless you have damaged the connection in the disassembly process (which is unlikely). Sometimes all you need to do is reseat the LCD connection and disconnect the battery for a few seconds to completely reset the phone. Another cause of this issue could be the LCD connector itself. Make sure it has not been ripped/damaged in any way. IF upon close inspection you find it has been damaged, a new LCD will then fix the issue :)
