Nothing responds anywhere
Every switch in the house is unresponsive. This looks like the worst case and is usually one of the cheapest: a whole-system symptom points at a single shared part, and the system has exactly one of most of them.
Read that again, because it matters when someone tells you the system is finished. A fault that takes out everything at once is, by definition, in something everything depends on: the network power supply, the bus wiring backbone, or the mains feed to the enclosure. Those are single points of failure, which makes them single points of repair. Widespread damage across many independent modules is rare and looks quite different.
Nothing on this page asks you to take anything apart. Every check here is done by looking, or by pressing buttons that are already on the outside of a wall plate. Opening the hinged door of your switchboard to see the breakers is normal and fine. Removing the panel behind them is not, and neither is unscrewing a wall plate or lifting the cover off an enclosure.
In Australia it is illegal to work on fixed wiring without an electrical licence, and the reason is not paperwork: the terminals behind a C-Bus wall plate can be sitting at mains voltage. Anything past the faceplate is a job for a licensed electrician, ideally one with C-Bus accreditation.
Are the wall plates themselves showing any light at all?
Look at several plates in different parts of the house, in a dim room. You are looking for any backlight or indicator glow, not for the lights working.
- Every plate is completely darkNo unit anywhere has bus power. That is the network power supply, its mains feed, or a dead short across the bus.
- Plates are dimly lit, flickering, or coming and goingThere is bus power but not enough of it, or it is unstable. Units drop off the network when the voltage sags.
- Plates are lit and reacting normally, no lights moveThe network is healthy and the switches are talking. Nothing is acting on the messages, which points at the output side.
- Other things in the house are dead too, not just C-BusIf power points or non-C-Bus circuits are also out, this is an electrical supply problem and C-Bus is a bystander.
If it started after a storm or an outage
Then take the blackout path instead: it has a different order of checks, because the most common post-outage faults are things that simply did not restart rather than things that broke. Go to it stopped after a blackout.
What this is almost certainly not
It is almost certainly not "the whole system has died of old age". Wired bus systems do not fail all at once from age; they fail one module at a time, over years. A sudden total outage is an event, and events have a single cause. If an electrician who does not know C-Bus tells you the whole lot needs replacing on the strength of a total outage, get a second opinion from someone who can actually test the network before you spend anything. What replacement really costs covers how to read that quote.