I had a look inside and as I thought the CD drive cables (both power and data!) had worked themselves loose. I reconnected them together, booted up.....and nothing, just a black screen. I was initially worried, but after rebooting again with both mouse buttons held down, the early start-up screen appeared (phew, I thought I had really broken it! :-)).
I will have a play around with the CD drive jumper settings, it may be something to do with that. :-)