Hi, I accidentally turned off my DS during the updating process. What should I do to fix it now that my DS can't read it?
Simply put, you bricked it. If you have no other Flash Cart, then you can't try unbricking it. In another word, you'd need to buy new cartridge.
http://forum.romulation.net/index.php?topic=61076.0 Down the bottom, under: "I stupidly updated my DSi or 3DS system menu before patching my card. Now what?"
It translates to : putting the needed firmaware installer on another flash cart, load it to the NDS (not DSi or 3DS), when prompted to start the installation process, eject that cart and insert the bricked cart then press yes to install. If done correctly it'd be unbricked. If done wrong, you might ended up with 2 bricked carts.