Strange WordPress issue :WYSIWYG editor can't switch between visual and HTML

We’ve got the latest version of W.P. For some reason, the WYSIWYG editor is not displaying the text properly and the editor is not responding to clicks…can’t switch from Visual to HTML

Has anyone encountered this ?

Have you made any changes to the css file(s) which are in use?

Do you have plugins installed, e.g. Secure WordPress? I’d try disabling that (or other plugins) and see if the issue still remains.

Check to see if the checkbox next to Disable the visual editor when writing is not checked in Your Profile under Users.