I was getting desperate because there were no responses to my plea for help, so I kept thinking (my computer wouldn't let me do what I wanted, so I had time to spare). Anyway, I have found the answer to my problem.
I reasoned that because it happened when I used Drive Image then it must be something to do with that program. So, believe it or not, I actually read the instruction manual!
There, Lo! and behold was the answer. At the opening DOS screen you click on OPTIONS and you can access a menu item for Hide/Unhide partitions. I must have accidentally clicked on the wrong button at an inappropriate moment.
I am publicly confessing my own stupidity in case it is of help to anyone else.
Thanks to all of you who would have helped if it had been your speciality.