It may sound strange but the first trouble-shoot is something that happens to me with the VGA gizmo on two occasions in about four years and twice with the composite cable on a different DC (which is more than likely NOT your problem), then the second one is usually the fix IF it is not more serious (I had this problem once before, and I was able to get it to work).
These options are based on the fact that you did use two DC cable methods.
1. Wiggle the connector and press it in firmly to the DC (being you used two DC cables, this is most likely not your problem).
2. Remove the top shell of the DC, then make sure the motherboard screw at the back of the DC are tight (do not over-torque). I thought this was weird at first, but when I did this about six years ago, my DC has worked fine since. Also, while you have it open, I'd unplug the power connector (the large white one with four conductors), clean it and then plug it back in.
3. It's dead, Jim!
One more note, the screw at the motherboard hopefully is the original screw as if it was replaced with a longer screw, it can short out or damage the AV connector port on the DC.