try this mate this worked on one of mine
the problem for me was tha the SD Card after puttng the 3ds back together somehow someway it would pop out just enough to not work so i put some tape around the Top of the SD Card and its been fine for years ............ if it is not this
did you update recently ? because if you did ..LUMA won't work so the 3DS won't boot ..so all you have to do is Update Luma and then you should be fine ...