If you want to restore the original firmware for some reason, you can follow the instructions below. Please note that this will only work if your Debian system is still working. If your Debian system is broken but the machine itself works, it is possible to restore the original firmware using a serial console.
You first have to download the firmware image from D-Link or Conceptronic and then unpack it. You can unpack it with dns323-firmware-tools (version 0.7.3 or higher):
splitdns323fw -k uKernel -i uRamdisk vendor-firmware
Now that you've extracted the kernel and ramdisk from the vendor firmware image, you can write them to flash:
dd if=uKernel of=/dev/mtdblock2 dd if=uRamdisk of=/dev/mtdblock3
When you restart your machine, the original firmware will start. The original firmware will offer you to format your disk.
Please note that simply starting the original firmware will destroy your Debian installation on disk (even if you don't choose to format the disk)!