Follow these two threads
http://forum.xda-developers.com/showthread.php?t=2432476
http://forum.xda-developers.com/showthread.php?t=2475045
If u still have Problem post here Will Help u .....
EDIT:
1.Turn off the G2 by holding down the power button and keeping it held down until the screen goes black, then continue to hold it for a few seconds after it goes black to make sure it shut down.
2.Plug the G2 into the computer via the USB cable while holding Volume Up at the same time to put it into download mode.