As a follow up to this post, I have found by way of Scary Yankee Chick's comment that Deb's computer problems were being caused by a bad update to the McAfee Antivirus suite. The update blocked all access to the Internet. I had to uninstall and reinstall McAfee and the problem went away.
Thee McAfee website has a workaround to reinstate network access and to force a new update to fix the problem.
