I had this problem after installing a clean version of WordPress, most of the fixes I found ask you to deactivate plugins or delete and reinstall entire directories, but there is a simple CSS fix for this.
By default the text color is set to white, so you just need to change the CSS code to set it to black. FTP into your WordPress installation and go to wp-admin > css, and open up wp-admin.css in a text editor. Search for the text "theEditor". Change the color value to black.
Refresh your browser (make sure that you're not loading the page from the cache, to do this on Firefox and Safari click refresh while pressing shift to properly reload the page). You should be able to see text now.