Electronic components don’t last forever. The reason for the massive heat sinks and fans to cool them down may offer a clue. Your effort to switch cables and determine the source of the malfunction is always first on the list. Sometimes it’s that simple, sometimes not. Assuming it’s not the source input, if it’s an integrated amp, you either have to send it in for repair or replace it. If not switch output channels to determine what component took a dump. As for me, I keep a low end, but respectable back-up integrated amp on hand just in case. Could be as simple as a tube just took a crap. Best of luck.
Ultimate stereo nightmare
I have a complex stereo system, having accumulated many various components through the years. I also have an impenetrable jumble of wires. Until now I’ve been able to deal with problems that arose. Now that I am elderly I can’t easily get down on the floor to assess things. I bought and installed new very expensive speaker cables (Audioquest Robin Hood), and subsequently found that my right channel was not working. After switching cables, the right channel was still not working. So the problem is somewhere in the system. I don’t know if it’s a loose cable or something else and have no way of tackling the jungle of wires to check it out.
I hired a company that claims they deal with these things, but there’s such complexity I don’t know whether they can deal with it.