Just came back home.
I write it down in case some people have same troubles.
RECAP: I switched between micro sd cards while in Hekate and got a freeze in Hekate menu then a total black screen and ZERO way to turn it on again.
I thought it was a total brick. Tried to hold the power button, charge, let it hours but nothing changed.
I remembered I activated “auto rcm mode” .
So I plugged the Switch on my computer to load a payload and the Switch booted on as usual. Just like before.
SO.
If your Switch freezes in Hekate and you have a black screen, do not panic, do not relove the battery, just load a payload on your computer.