I'm not sure if restoring a NAND backup from another 3DS would cause a hard or softbrick, although I'm pretty sure it would cause a hardbrick.
You can fix a soft brick by updating your 3DS through recovery mode.
You can fix a soft brick by getting a hardmod (and a proper NAND backup. If you do not have a proper NAND backup, your 3DS would be permanently bricked.)