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Try this:
Pull battery. Hold power button down for 30 sec. Re-install battery. Hit the power button as fast as you can,all the way down ,twice. Make sure you let go of it. It should boot into you ROM, if not Download "Screencast" onto a PC.
When you have Screencast set upon PC make sure you have the proper LG drivers installed.
Screen cast is going to allow you to see the recovery screen menu,even though you can't see it on the phone it will show up on the PC screen.
Put the Xionia v1.21548.6 recovery and the ROM (and gapps if a gingerbread ROM) on the root of the sd card. The recovery needs to be unzipped (will have .img on the end). The ROM (and gapps if needed) needs to be left zipped.
Pull battery again and hold power button for 30 sec. Re-install battery. Hold home+volume down and then the power (all at the same time ). Plug phone into the PC. It will take a couple of min. When the recovery menu is displayed on the screen proceed ( there is a little bit of lag between what you do on the phone and the display, so take it slow)
Go through the install procedure for the ROM (and gapps if necessary) which can be found in the All Things Root sub-forum, making sure you wipe and format everything that is required before installing the ROM (and gapps if necessary).
Once booted up into the new ROM, reboot a couple of times then go to the market app and sign in to your google account. Then download a terminal emulator.
Open up the terminal emulator and type in su (hit the return key). Then type in the rest of the code to install the recovery on the your phone making sure you spell the name of the recovery exactly as it's spelled on the sd card.
You will have to find the code for installing the recovery that is used on your phone model by looking into the All Things Root sub-forum where I've moved this for you.
