Hooray. A bit of internet research revealed the possibility our mac wasn't dead after all. We pulled out plugs, held buttons down, put plugs back in AND it worked!!! Just a temporary blip and not the disaster we had assumed. Neither of us realised quite how much we depended on the computer until we didn't have it any more - the relief is great.
Thank you very much to everyone who left kind words and advice. The very first thing I did was back up. I do have an external hard drive, I just hadn't backed up for months. I will try and be more assiduous from now on.