Thanks Simon.
Yes I backed MOST of the files up barring the one I need. Typical!
I stopped the optimisation process quite early in the process altho it had been running for 12 hours with little progress on the progress bar.
I had tried booting from a system cd as well as connecting through a network from my other Mac. That way I can see my user directory but no shared folders inside it.
So essentially the files are on the HD but the main startup disk can’t be seen usually the StartUp from system Preferences.
If I rebuild the catalog file with Discutil will I lose everything off the HD?