[tonyvdb]

There will be a button on the front that will say I believe "display" once you push the button the Onkyo should show "HDMI out on/off" or something like that. (I am asuming that your have it connected to the display with a HDMI cable)? you will need to turn it "on"
If that does not fix it try connecting the Svideo or Component video output of the receiver to the display and see if you get a picture then.

 

[tonyvdb]

Do you use the tuner in the TV to watch TV or are you using a PVR or other external tuner? If its external Its sounding like the output for the video has either gone bad or the settings in the user menu of the unit is outputting via composite, SVideo or component. You may need to check that.