Thanks for all your help but I'm not making progress. Here's my plans: 1) I'm going to experiment with Terminal a while.2) Then, if that fails, I do have the ability so save, one by one, individual files on to the external hard drive. It'll take time but I don't have an enormous amount to save.
3) Then, I'll try to reinstall. 4) Then, if that fails, I'm taking it to the shop.
Thanks again.