To confirm, this is the standard BeagleBone A6 (with no monitor output)
I’ve been using this board for a long while as an ssh server. One day I couldn’t reach it.
With no SD card inserted if I plugin just a USB cord running to my laptop I only get the pwr light lit and the reset button makes no difference to this. If I plugin a 5v PSU I then get the heartbeat LED but nothing more. If I then plug the USB cable back to my laptop I get over /dev/ttyUSB0:
The SD card passed fsck -f and looks ok from a cursory look. Does it look like a power surge has broken this board in some way?