Your DVR may be set to output at Full HD (1080p) resolution, and the display you have plugged in does not support this. This is common on VGA monitors 19" and below, and older 'HD Ready' televisions.
You should set the DVR to XGA or 1024x768 resolution, which is compatible with nearly all screens.
Cobra: Look for the two small switches next to the power connector. One will be pointing to 'HD', flip this down to 'XGA' and reboot the DVR.
Odyssey: HDMI/VGA resolution setting is available in the Display Output menu - if you have no other means of viewing the menu, you may have to attach a BNC monitor temporarily.