Thanks to @the_laser and @vovanx500
To PREVENT a Hard Brick, READ THIS ENTIRE POST!
Take part in the poll ONLY if your phone cannot be revived
ie. Pressing Volume Up + Power does NOT work and you can't get into flashmode. (If that works, it's not a hard brick)
It's NOT a hard brick if you can get into flash mode.
It's NOT a hard brick if you see the Sony logo.
It's NOT a hard brick if you see ANY life on the screen.
It's NOT a hard brick if you get a led colour that isn't red.
And if it's NOT a hardbrick go HERE to fix it.
Or just flash an ftf from HERE and wipe all / exclude nothing ie. Go with Flashtool's default settings.
But if it's really hard bricked and if took part in the poll... join the club...
...The Brickfest Club
Warning 1. Use LATEST Flashtool! (v0.9.11 is DANGEROUS when flashing 4.3 ROMS)
Warning 2. When updating from a 4.2.2 ROM (*534 or *257 ftfs) to 4.3 (*290/*136 ftf), in Flashtool, DO NOT flash only the kernel and baseband from your chosen 290/136 ftf (the last step).
Flash EVERYTHING EXCEPT SYSTEM!
Read this post in which I elaborate as to why.
Basically, there's a 4.3 (290/136 ftf) kernel incompatibility with some aspects of 4.2.2 (534 and 257 ftfs).
I strongly suspect a mismatching ELABEL or BOOTBUNDLE to be the reason in most bricks. MOST likely it's the BOOTBUNDLE.
There's also some funny business regarding the battery level being misread.
So always be sure, when flashing parts of a 290 or 136 ftf, you flash, at the very least:
KERNEL, ELABEL, BOOTBUNDLE, AND BASEBAND so that there is no risk of a mismatch.
Let's refer to it as the KEBB group. Easy to remember :P
How to update from 4.2.2 ROMs (534 or 257 ftf base) to 4.3 ROMs (290 or 136 ftf base)
1. Go to recovery and do a factory reset.
2. Flash the 4.3 ROM
3. (Optional). Reflash recovery just in case it got wiped from step 2.
4. Using the LATEST Flashtool, select a 290/136 ftf to flash.
5. In the last box, ONLY EXCLUDE SYSTEM (ie. select only SYSTEM and leave all other boxes unchecked. In this area, items with NO checkmark get flashed.)
And it seems to happen even when the user does everything right.
So I think it would be a good idea to try and figure out a pattern to it and determine how and why it happens.
So if you got bricked, post the pertinent details
Phone model - C6906
Method used - FlashTool version - 0.9.13.0
FTF file used - C6906_14.2.A.0.290_Generic CA.ftf
Battery Level - Quite full. Over 80% for sure.
How it got bricked - Flashed the Canadian Generic 290 kernel on a Canadian stock rooted 534.
Original OP below
So I was experimenting with getting 4.3 rooted or at least have the latest bells and whistles on a properly rooted rom.
I thought that I'd try mixing and matching things maybe flash a new 4.3 stock rom, do a backup, flash a 534 and root it and restore 4.3 apps.
Not the best idea but I figure no big deal if it didn't work since I can just flash a stock ftf, right?
So I was tired... and I carelessly flashed a 4.3 stock kernel on a rooted 534.
I unplugged the phone and it's totally dead.
Doesn't power on, no lights, can't get back into flash mode.
Power + volume up = nothing. No vibration.
Tried the red hard reset button... still nothing.
The phone was pretty much fully charged at the time but leaving it plugged into a wall charger to see if anything good happens.
edit: after leaving it plugged into a wall charger for about 24 hrs, the red light started blinking but no matter what I do, I can't get into flashmode.