After taking damage from a zombie your screen fades and becomes grey and also blurry. After healing with a bandage or a rag the greyness goes away but the blur is still there.
-The Solution to the blurry screen-
Press the Esc key and click on the "Configure" menu. Then proceed as if you are going to change your settings. Click on the "video" menu....the screen should then fix itself and become normal again.