Well, I did what the instructions said
and it has worked - I can now access the things I couldn't a few hours ago. But the question remains ... when will it happen again? I'll get my 'fix-it' guru Hank to have a look ... but at least there is solace in the fact that, in emergency moments when Hank isn't around, I can type in questions to Google and find answers that enable me to fix Firefly (name of my computer) ... until the next time. (I can't believe that even I am sounding catty about PC's now).
No insult to you, Firefly.
Even though you've given trouble now and then over the past 2 years or so, it's never been anything horrendously drastic, has always been fixable and on your good days (which are at an approximate 96:4 ratio with the 'bad' days) you've done good work with me.