[Kindle Fire Q&A, Help & Troubleshooting] darrellburk:
How to recover when a bad bootloader flash puts your Fire completely out

I had this same problem, and it came from flashing u-boot.zip instead of the u-boot.bin it contains.

Once flashed and rebooted, the Kindle Fire became completely unresponsive. No power/charge light, USB port appeared completely dead. I called Amazon support and they sent me a new Fire. But I just couldn't accept that it was unfixable, so I kept trying. And I figured it out.

Here's the procedure:

Disconnect the USB cable.

Remove the Fire's back cover.

Disconnect the battery. This step is critical! It ensures that the CPU is completely powered off.

Have your USB cable plugged into the computer, but NOT into the Kindle yet.

Ground the "USB boot" pad on the Fire's motherboard, as described elsewhere (the USB boot shorting trick). Keep it this way for the next step.

Plug the USB cable into the Fire. At this point, the Fire has been turned on in USB boot mode and is being powered exclusively by the USB port. You can now stop grounding the USB boot pad.

Now reconnect the battery. This step is critical!

Use usbboot as documented elsewhere to send a good bootloader via USB and boot with it. You should end up in fastboot mode. AND your power light should be back on, probably green! NOTE This only boots with the bootloader you sent, it does not flash it. The bad bootloader is still installed.

Now use fastboot to install a good bootloader, and a recovery if you want. Or use a firekit script...

Finally you can reboot for real. Make sure it will really reboot before you put the back cover on. You may have to change the bootmode back to 4000.

You now have a working Kindle Fire again.

Why it works. This is conjecture based on my findings:

First, as long as the battery is providing power to the Kindle, the CPU is not powered completely down, even in this case where the device seems totally dead from a bad bootloader flash. Therefore, grounding the USB boot pad does nothing, because the CPU only honors this on powerup. Disconnecting the battery and USB forces the CPU to power down. If you then ground the USB boot pad and connect the USB, the CPU powers up and goes into USB boot mode.

It's possible that if you wait a long time (maybe many days) so that the battery discharges to the point that the battery protection circuitry stops all further drain, you might get the same effect without disconnecting the battery. I didn't have the patience to test this.

Second, the bootloaders I used (two firefirefire versions) apparently check whether the battery is connected, and halt if it is not. I suspect this is true of most/all available bootloaders. So even if you get into USB boot mode and send over a good bootloader, once the Kindle starts booting with that bootloader, if it finds that the battery is not connected, it quits: it never gets into fastboot mode. Hence the "waiting for device" messages noted earlier in this thread. Therefore, as soon as you have gotten the CPU in USB boot mode, but BEFORE you've run the usbboot command, you need to reconnect the battery.

The only thing that remains is to listen to the angel choirs sing.

And wonder why you had to brick the replacement Kindle that Amazon sent--and break some of the tabs on its back cover in the process of fixing it--in order to fail at proving to yourself that you did not cause the problem in the first place. And decide whether you're glad that you now own TWO Kindle Fire's with loose back covers... :o