Oh dear Lord - what a crazy two hours I've just had. My computer spared out, shut off, I could not restart - I called Apple Support and got some guy who really didn't know much, so he couldn't get anything working and made me an appointment for Friday. Then on a hunch I called back and got someone who was smart and nice, and he had me do other stuff and we managed to get back on. We were screen sharing when it shut down again. So, we went into 'recovery' mode and tried to do a drive repair and it wouldn't do it, which means the drive is corrupt or going bad or whatever. So, in it will go on Friday, although we did manage to get back to the desktop and I am working from there right now. But smartly, I put my main folder onto an 8gig little memory thing so I can work from that on my MacBook Air if I have to. Crazy stuff. I asked if I should put it to sleep when I'm not working and he thought that was a good idea so that's what I'll do and hopefully it will be okay for a while.