Editing Posts in 'Edit Page' in the Visual tab are invisible

When I add a new post or wish to edit an old post on my blog, in the ‘Edit Page’ mode, the visual tab is invisible. In fact, there is content in the blog post because the page displays well when published, but I can’t see anything in the Visual Editor tab, when I attempt to edit the page or create a new one. Strangely, I can read the HTML tab, but nothing is readable in the Visual tab.

This has only happened in the last few days. At first I thought my internet connection was not loading properly, but it seems a more serious problem? Look forward to your thoughts on how to fix this problem. Thanks, Simon.