I couldn't get online all day yesterday, presumed it was a problem with the ISP. Tried again this morning, still no luck. The only thing I could think of was a Windows update was installed on Wednesday night. Before phoning the ISP I decided I could try shutting down Zone Alarm to see if that helps.
Wow, I got online! Did a quick search and came up with this:
http://technology.timesonline.co.uk/tol/news/tech_and_web/article4304820.ece
So thanks to Microsoft leaving a security hole in Windows for a few years which Zone Alarm must have been using in some way I wasn't able to get on.
Oh well, fixed now. Hope no one else is having that problem.