It sounds to me like either the New 3DS's lithium-ion battery has run out or is not connected. If a blue light doesn't come on, it usually means there's no power supplied to the system itself.
But if the battery is fully charged and the light still doesn't come on, then I'm really sorry to say, that your New 3DS's MCU has been destroyed (in other words, a component on the N3DS motherboard). And if this really is the case, then I'd say someone installed an incorrect DevMenu.cia for the wrong system, as there are different ones for each system. There's ones for PANDA units (Original 3DS), and there's ones for SNAKE units (New 3DS). If someone was trying to install a DevMenu.cia, it needs to be for the correct system.
But if it really was a config.cia, it's quite possible that one from a O3DS was installed onto a N3DS (which can destroy the MCU).