Did your clicking finger twitch and do a double-click by mistake perhaps?
Thank you for your further response.
I don't think so. My old gnarled hands are such that I cannot do a double click. Instead I must right click, and then click on the "open" option.
Even going into the mouse properties and slowing the double click feature to it's minimum speed, I do not manage a double click, but rather produce two clicks.............not the same thing.
However, because of the same hand problems, anything can happen so I may well have upset the system by doing a wrong click somewhere.
Anyway, obviously not a serious prob. so will put it aside and see what happens at first go tomorrow.