@rafciuk When the connecting screen appears, Hold the reset button in for around 5 seconds. it takes some practice but the grey is not dead just soft bricked.
Another option is to try using arduino to upload a blank sketch before trying M5Burner again.
I have accidentally uploaded the wrong firmware before and had to reset in the above manner.
The device is not dead it just that the core and core2 use different screens and so you cant see what it is doing.