Previously working, now failing to boot

I have HomeAssistant installed in a Virtual Machine (VMWare ESXi, in case that matters). I haven’t needed to do anything to it for several months and I have a dreadful memory for such things but, according to some scrappy notes I made at the time, I understand it’s a “hassio” installation. It’s been working away happily without problems but now (possibly after a power outage) it’s failing to boot properly and ending up on what appears to be an “Autoboot” menu screen. The options don’t display but I evidently have “system 0” and “system 1” boot options. These both launch some sort of boot sequence that end up restarting, and I haven’t found a way to freeze the process so I can read any error messages.

Exploring the “Shell” option I found a “bootchooser” command that produces the following:

HAOS-boot:/ boot
Booting entry 'bootchooser'
bootchooser: No valid targets found:
B
  id: 2
  priority: 20
  default_priority: 1
  remaining attempts: 3
  boot: 'system1'
  disabled due to remaining attempts = 0
A
  id: 1
  priority: 10
  default_priority: 1
  remaining attempts: 3
  boot: 'system0'
  disabled due to remaining attempts = 0
Booting entry 'bootchooser' failed
Nothing bootable found

… but I’m afraid that means little to me and I haven’t yet managed to find anything that sheds much light on it. Can anyone suggest anything I might try from this point to attempt a recovery without reinstalling from scratch?