The PC or touchscreen that ran it has died

This is the classic orphaned C-Bus situation, and the first thing to say is that it is far less serious than it feels. Your switches almost certainly still work. What you have lost is the ability to change anything.

People assume the dead computer was running the lights, and that without it the system is finished. In most installations that is not how it works. The switching logic lives in the C-Bus units themselves, on the network, and it keeps running whether or not any computer is connected. The PC was there to program the system and, in some houses, to provide a nice interface and some extra automation on top.

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.

Which of these is true right now?

What actually lived on that computer

The project file
The master record of your system: every unit, every group, every scene, with the names someone typed in when it was commissioned. Losing this is the expensive part, not losing the computer.
Programming software
C-Bus Toolkit, and possibly PICED for touchscreen logic. Both are Clipsal software and can be obtained again. This part is not the problem.
Interface logic
If you had a touchscreen or a PC front end, its pages and any automation written into them lived there. That is the part that genuinely disappears with the machine.
Nothing about switching
The wall switch to light relationship is held in the units on the network. That is why your lights still work.

Do not throw the machine out

Even a dead PC usually has an intact hard drive, and the project file is on it. Recovering a file from a drive that will not boot is routine and cheap compared with rebuilding programming from scratch. The same goes for old laptops in cupboards, USB sticks in the folder of house documents, and CDs from the original handover. Before anyone quotes you for reverse-engineering the system, spend an hour looking: the lost project file page lists everywhere worth checking.

Different symptom? Back to the diagnoser