First of all: figure out why it is not booting anymore: have you overwritten the bootloader? Have you flashed your own FW from start of flash (0x08000000)?
When you connect external debugger: make sure 0x08000000 ... 0x08040000 is never touched!:
if you overwrite this region - the bootloader is gone. And also: the PMIC init is gone.
If you flash your own FW from 0x08000000 and it does NOT initialize PMIC by your own FW properly - all is bricked.
Try this:
- try to connect to board via external debugger
- watch the target voltage reported: if too low (in mV range) - nothing will work, even debugger
cannot connect or read/write flash memory fails
Try to figure it out: potentially an issue with PMIC and the missing power on the board (PMIC not initialized anymore).