Possible causes:
- If custom firmware is installed, boot.firm cannot be found on the SD and no backup copy of this file exists on CTRNAND
- You did not properly connect one or more ribbon cable(s).
- You accidentally damaged one or more ribbon cable(s) and/or connector(s).
Make sure
boot.firm is present on a FAT32 formatted micro-SD inserted to the N2DSXL if CFW is installed.
Else you have no choice but to open the console again and double check all ribbons and connectors. (Maybe upload good, detailed pictures).
Not finding hardware components makes the 2|3DS consoles mostly shutoff immediately, sometimes with a popping sound.
The blue LED slowly fading off is in general related to missing/not found
boot.firm. But it could be both.
Troubleshooting this can be a pain.