A guy I used to work with in the engineering department of a public broadcasting facility used to appraise situations thus: "There are only two possibilities, either it's the supply or the load". That well sums up ESL problems. If it's the power supply--a pretty common failure point for an ESL--and the step-up is delivering signal to the panel, you might still hear something, albeit very distorted, if you put your ear on the panel. It would be pretty unusual for a panel, new or old, to suddenly deliver no sound. If you replaced it a couple of years ago, I would say check all the connections you made. If it's a Molex or other type of plug, make sure the wire didn't get caught up somewhere, causing the plug to eventually pull loose. I would say re-checking your work when replacing the panel would be the best place to start.
When I replaced a panel on my CLS II I carelessly pinched a wire when putting the electronics enclosure back together. I had to find some high voltage insulating tubing to put over the boo-boo. Something like that, undetected, could work for a while then cause trouble.
