How to recover bricked ESP8266


I had old very old firmware on my ESP-12 module, so, decided to upgrade.
I got the ESP8266 firmware from AT | Espressif Systems
I successfully uploaded firmware using Flash Download Tools (Tools | Espressif Systems).

Then I detached CPO0 PIN from GND and reboot the ESP8266.
After that, I completely lost it. :frowning: ESPlorer doesn’t see the module at 9600 and 115200.
When I connected again GPO0 PIN to GND to upload firmware again, the Flash Download Tools doesn’t see the module.

So, I don’t understand what’s a problem. I need to update firmware for several other modules and afraid to brick them also.

the update can't be the reason. the bootloader is read-only in MCU and the firmware is flashed to external flash memory. the gpio 0 LOW activates the bootloader only at boot