Why exactly? Asking because there is a guy on another thread that had a similar problem, and thought them same: If AutoRCM was on, he probably bricked his Switch.
So, basically you didn't even manage to downgrade, right? Try to turn it on, and get your ear close to the fan exit. Sometimes you can feel or hear the fan spinning.
It looks like a new type of brick, probably related to RCM or AutoRCM