And welcome to LQ!
Impossible to tell whether it was switched off, reset was pressed
or there was a glitch in the power. But it definitely went down
without a warning. Might even be a box set-up to reboot on a kernel
panic without human intervention. For us a dying motherboard (capacitors
had come of age) did things like the above, too. Just reset itself at
random times. Heaps of possible reasons, and, as stated above, all you
can tell is that it went down without a warning.