I had the same problem with my Xperia 5 ii after I re-locked the bootloader using fastboot (
fastboot oem lock).
I could fix the problem with Xperia Companion. Here is what I did [maybe labels are not exactly correct, my interface is in german]:
Pre-requisites:
- I did not use the original Sony USB cable, but a third party cable from Anker, also worked
- Disconnect device from computer
Steps to fix:
- Install and start Xperia Companion
- Click "software repair"
- On the bottom click "My device is not detected or is a Smartwatch 3"
- Choose your device, click "Xperia smartphone or tablet"
- Follow steps on the screen:
- Disconnect and power off device (should already be)
- Press and hold volume down
- Connect device via USB cable, still hold volume down, the notification LED will turn green
- Repair starts
- Wait about 10 to 20 minutes until repair is finished
- Screen will stay black and notification LED will stay green during the whole procedure
After these steps, Xperia Companion showed a success message and my device turned off. I disconnected the cable and started it (press and hold power button for about 1 second). The device first only turned on the notification LED in green, screen stayed black. After about 5 to 10 seconds, the device turned off again. I started it again (press and hold power button for about 1 second) until it vibrates, and it started normally.
Of course the device was reset and all data was wiped. For me this was no problem, as I anyway resetted it before.
Hope that helps
