edit - problem solved by reinstalling.
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Firefox announced it needed to restart having just updated itself, showing a 'restart' button, and presenting a message saying it would be back in a sec without any inconvenience to the user.
Pressed the provided restart button.
Firefox disappeared, didn't return.
Tried manually restarting Firefox, 'command not found' or something similar.
Tried to reboot, system now provides two messages at the top of a black screen, just after the lubuntu loading graphic starts:
snd_hda_codec_conexant hdaudioC0D0: vmaster hook already present before cdev!
bluetooth: hci0: reading supported features failed (-16)
It then hangs completely.
It's a dual boot machine, tried booting into Windows, everything OK in there.
Booted lubuntu from a USB stick & managed to load a backup copy of my firefox profile, that's where I'm up to now.
Someone suggested the bluetooth error might not be the actual cause of the hang, simply the last fleeting minor error noted by the startup sequence before something in the display setup crashed?
I have some access to the filesystem on my 3 drives, although some seems to be locked read-only..
Any ideas?