Not sure if there is something selected here -- but I have changed the font and whatever I type in -- a blurry letter comes out. I have tried just doing a new composition and same thing and can get around it.
Reset the character panel using the layer panel menu at the top corner.
Check the font size and the composition viewer settings. This looks to me like you have selected a font that is about 2 pixels wide and the zoom factor of your composition is about 800%. Set the zoom factor, you'll see that on the bottom far left to 100%, make sure the comp resolution is set to auto, then set the font size to something big enough for video. What you are seeing is completely normal when you use fonts in an app that renders pixels.