"Update (07/07/26): The Reddit user with the inoperable Steam Machine was able to get it working again the next day by leaving the device unplugged and then turning it on again and leaving it, while a Valve employee reached out with advice for anyone facing similar issues to get to the bottom of the issue more rapidly.
Interestingly, the Valve rep mentioned that the lightbar codes are reversed from what they intended, meaning the code that ought to indicate "GPU failure" instead means "memory training failed" - which explains how leaving the machine unplugged (to clear the CMOS) and then starting it again and leaving it alone fixed the issue. The Valve support doc doesn't seem to reflect this at present."
Steam support reacted to a request by a user with the instruction to unplug the Machine, plug it back in later, hold the power button for around 6 seconds to launch the recovery menu.