The data cassette player would not power up and there was no sound outputted.
There is a big 1.5A fuse inside labelled F1. This supplies the +9v to pin 28 of the SID chip and is also used in powering the cassette unit.
Replacing this fuse brought everything back to life.
The C64c boots up fine without this fuse. I have been using an SD2IEC adapter to load games and didn’t realise at the time that the cassette unit was not working either.