I have a similar problem. I powerd my M5Stack core black from a Stepmotor module and was not able to connect with the WLAN for programming. So I connected a small powerbank to the USB-C of the M5Stack. After resetting the display stayed black. Using a power adapter for the USB-C port also did not change anything. The M5Stack did not start. What I found out was that the powerbank did not deliver 5V correctly ( I measured 3.7V). May there be a conflict with the charging circuit when the Core is powerd over the Stepmotormodule (12V) and a voltage lower than 5V from the USB-Cat the same time?
Some weeks ago my M5Stack grey stopped working when i tested a LEGO+ module.
Now I have a black and a grey module and both displays keep dark and are not working.
Any ideas?