Flowdeeps My BBB won't boot all of a sudden. I get a blue power light but no activity.
hmm my gut reaction would be: failed CPU. The blue "PWR" light is managed by the PMIC (power-management integrated circuit) while the other LEDs are managed by the CPU. If these do not come up, then it may well be that the CPU has failed.
Flowdeeps I've read about a rogue UART character in a few blog posts but am unsure how it relates to fixing the non-booting issue.
This would only be occasional, only happen on older BBBs with old bootloaders on the eMMC, and should be fixed by simply powering it off and on again.
Flowdeeps I've tried reflashing the EMMC with the latest default Ångstrom image for the BBB but I can't get the lights to flash when holding down the user boot button.
In order to flash the eMMC you need the device to actually boot. Perhaps you mean "reflashing the SD card"? The "latest default" Ångstrom image for the BBB is dated 2013: https://beagleboard.org/latest-images . You should try the Debian Stretch IoT image, or the Bela Image: https://github.com/BelaPlatform/bela-image-builder/releases .
The only way to check if this is a complete failure of the CPU or something going wrong during boot is to remove the Bela cape and connect a serial device to the debugging header of the BeagleBone (see here). Note if you have another BeagleBone, or an Arduino or anything with a serial port on them, this could save you from buying an FTDI cable.