I would start from scratch and flash it over ethernet with a new image.
Assuming you have a LAN with some device that will assign an IP address using DHCP.
Power off the box, then hold the standby button in while you reconnect the power supply, after around 10 seconds flash should be erased.
I think the DM-800 displays its IP address on the LCD screen when you do this.
Once you have the IP address, you can connect using a webbrowser and load the new image to flash.
EDIT:
Flash Your DM800 - The Easy Way