After reinstalling original front assembly, backlight won't work correctly on LCD

[jiniej]

Title explains it. I took off my front assembly to put it on my friends 3Gs so she could check her texts. Now, both of our LCDs aren't working. That is: As soon as you activate the LCD (whether it be wake it from sleep or restart/power on) the backlight will go on for a split second (at what looks to be full power) then turn off completely, so the LCD is still displaying pixels but the backlight is off so the image is extremely faint. To me, it looks like a software issue, but I have tried every possible thing I could think of to fix it software wise, to no avail. What did I do???? I look like a complete tool now that I messed up both our phones...

[GnatGoSplat]

Were the phones on when you did this? If so, you may have blown the coil. Search Google for 3gs coil and you should get several relevant hits. You should be able to check it with a multimeter if you have one, it should have continuity through it.
If the coil is still good and you have continuity all the way from the flex connector to the coil, then it's possible if you weren't careful with the flex cable, you may have damaged it internally and need a new LCD.

Have you also tried rebooting by holding down power and home for 10-seconds? Or trying a restore?