Thank you for your reply Yer_ Man.
I was browsing the internet when suddenly I got a Ransom demand telling me that I was locked out of my computer, not to attempt to close down my laptop but to ring a specified number to unlock it. I tried to close down but PC was totally locked. A window appeared after a short while with the option to " leave this page " which I actioned . My laptop has been operating normally since.
The only fear I have is that this hacker might have downloaded, during this sequence of events, Spyware. I am reluctant to use my credit card at the moment on the internet as a result.
Your comments would be very much appreciated.