Something might get screwed up bad enough that your phone will no longer boot. Holding power won't do anything, not even give you a vibration or turn on the LED in the power button. Connecting to the computer only shows a QHSUSB_DLOAD device connected. This is a hard brick, but not a hopeless situation.
Things to try:
- Remove the battery and disconnect the phone from the computer.
- Hold down the power button for about ten seconds, then keep holding it while replacing the battery.
- If your phone does not boot, remove the battery and repeat step 2.
- If it does boot, recover your data, then try flashing the phone back to stock. Otherwise, try PC Companion.
PC Companion (aka Xperia Companion)
- Download the software:
- Download link coming soon, sorry! Hit up /u/Antnommer if this still hasn't been updated and you need it.
- Install and run PCC.
- Boot your phone into the OS and connect it to the computer.
- Select to reset your device, and follow the steps.