Keep in mind I only have what you tell.
I had this happen on a machine that the user had been updating video drivers. Also I've seen this as Windows runs out of memory so with that many tabs open, it's not a sure thing.
Try this. Click somewhere in the web page and then try Control+A and see if the highlighted version is readable. I had some old web site that was just bad and had to use this trick to read the page.
So in short, you must know your PC. That is, know that you have the resolution set to native as well as the supported video drivers.