Chosen Solution
Machine boots, but no USB works to plugin KB or mouse. Can’t boot into safe mode either, holding shift doesn’t work even earlier in boot sequence. Just in case there was dust in USB sockets, tried vacuuming them out. Seems this is likely to be a hardware issue? Actually pretty sure it is now. Tried various different USB devices and, oddly, extension cables as recommended some places, but even a USB hub with power indicator does not light for this machine.
Let’s give this a try! Locate an Apple Bluetooth keyboard and mouse/trackpad The system should respond to these if the Bluetooth port is turned on. Make sure they are turned on before you start the iMac. If they work restart the system, this time press the D key to enter into the onboard diagnostics. Let us know the error message. Reference: Mac startup key combinations If that worked otherwise this will need deeper diagnostics at the logic board level or the swap out of the logic board. If you need these ports. You might be able to get a USB-C hub which offers USB-A ports to get around the failure.
