It's happened to me twice over the last several years. 1 time I just hit zoom and set to 100%, the other time I had to do ctrl+a and then set the zoom to 100%. I couldn't just click on the 100% icon, I had to go into the Zoom and at the bottom where it says percent, somehow it was showing 10 -- I changed it to 100.
If neither works then I have no idea...