Switch won’t boot up

This question is getting pretty old, so I don't know if you've fixed it by now, but I can try to help. Without some more detail, my answer will be more general, but I hope it will help you anyway.

Did you make sure to have the same motherboard model on both consoles? The Nintendo Switch (not OLED or Lite) comes in two hardware revisions, the original one where the mainboards have product codes HAC-CPU-XX (XX being replaced by either 01, 10 or 20, depending on the manufacturing date, but they are essentially the same), while the most recent hardware revision (codenamed Mariko) has a product code of HAD-CPU-01. You can find this product code on either side of the motherboard, on the same border than the USB-C port. Basically, HAC and HAD motherboards are not compatible between them, and you have to replace the one you have by a similar one (HAC with HAC and HAD with HAD). If you do not remember which model you originally had, you can look at the serial number of your console, located under the barcode on the bottom of the case. In particular, check the second letter: if it's an A, you have the original one, if it's a K, you have a Mariko one.

Also, make sure you plugged the eMMC back correctly. This might sound obvious, but better to check anyway.

I hope this helps!