www.wysiwygwebbuilder.com

For some reason an important page has been lost from my computer right after downloading the very latest version of this program.
The complete file is however still operational on my isp and I can redirect the site links to it.
My question is how can I download/copy this page back into my WYSIWYG program on my computer?

I doubt that this has anything to do with the update.
But maybe your project uses extension which are not installed? If an extension is missing then the page cannot be loaded.
The software should display a warning about this when you try to load the project.

You cannot load a php page back into the software, because WWB is not a code editor.
But I think you should focus to find out why the original file could not be loaded.
Maybe one of your backups still contains the page?