Good question. If you cant see the menu then it may be defective.
What type of connection are you using? HDMI, S-video,....
Have you tried it on a different TV?
If you are able to get it to work I would suggest going through these steps to update the firmware.
In the menu of the 750, go to settings, system, and system reset.
On the next screen click on "reset".
After it resets, it will restart the 750 and you will need to go through the setup wizard again.
In the menu of the 750, go to settings, system, and update device.
If this does not work you will have to update the firmware manually.
You will need a usb flash drive.
Go to this website: ftp://ftp.dlink.com/medialounge/DSM750/
Download this file: dsm750_firmware_10209.zip
Unzip the file, put it on the flash drive, and insert the flash drive into the 750.
The 750 should automatically see the flash drive under "usb direct".
You should see the file. Click on it and I think it takes you through the install process.
Let me know...