I've just had exactly the same experience with my early 2018 HomePod as the OP (completely dead, wouldn't power up), and after a visit to the Apple Store (unsuccessful), then phone calls to Apple customer support, I have been told that my HomePod will be replaced with a new unit under my Consumer Rights. It was a bit of a battle to get to this point, but my advice is to stand firm and demand a replacement. The Mac Genius in the Apple Store confirmed to me that the HomePod is not a product that is designed to be opened up/repaired, and suggested that I buy a new one at full price. That's when I escalated the issue with Apple customer support.