My pbp is a sleek, black, flat brick, and won't boot from any kind of media

[bkenn]

No boot SOLVED
My experience for what it's worth:
I went through all the suggested fixes (described in a previous post: flashing zeros to SPI, checking daughterboard cable, etc) and finally started over. I had tried so many approaches, that I was losing track. I used a second computer, a 2gb HP celeron laptop running puppy Easy OS on a usb stick.
I disabled the 64gb emmc and removed it. I had already flashed the SPI with zeros. That left only the sd slot for booting.
I downloaded balena-Etcher on the HP, unzipped it, and changed permissions to allow execution. I downloaded the Manjaro ARM KDE plasma... 21.06 . I put a 32gb Kingston HC1 "Canvas" sd into a usb multipurpose reader from IOGear Model GRR304SD and put  reader into a USB 2.0 port. I used Etcher to flash the 21.06 file onto the sd card.  After the file was flashed (5 min), Etcher ran a validation check which failed. I redid the flashing, and the validation check said the flashed image was fine.
So, I loaded the Kingston sd card into the bare bones PBP, held the power switch down for a minute, clicked the tiny reset button on the motherboard, pushed the power switch for a few seconds, AND, for the first time in two weeks, the PBP lit up and went into action.
One down, one to go.
I removed the sd card, enabled the emmc ,  pushed the power switch and got nothing. So:
I loaded the emmc card on to an Ordroid USBEEMMC (Olympian LED) reader that I bought for the purpose, and put it into the 3.0 USB port on the HP. When mounted, I looked at some of the files and found that I had previously flashed the Manjaro KDE plasma...21.07 file, the one meant for intel machines.  Using Etcher again, I flashed the  ARM 21.06 file that  worked on the sd card.
I reinserted the emmc card in the PBP, and it worked fine.  Put the sd card in as well; and that worked.
So, it was all my stupidity. I highly recommend Etcher, the Ordroid USB-Emmc reader, and the IOgear USB reader. I used the pricier sd cards to be safe. If I have trouble in the future, I would not flash zeros to the SPI  until every other option failed. It took me days to do and I'm not sure that it was needed.
Loving my PBP.

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This is another NOOB who was reading your problem trying to solve mine.  This post did it!  A simple problem that looked like I was going to have to pop the back and get tools, etc.  I put the wrong OS on my SD card!!!!!   That is why it would not boot.  Sometimes - maybe most of the times - it can be the simple things that mess us up. Especially me.