Look up the product number on the back of the monitor on the Dell site. Find out what the screen resolution is (1280x1024, etc...). Press WINDOWS-M to shrink any open window, right click on the desktop, select properties, and change the screen resolution to match.
You can change the size of the font on your browser window to compensate for the smaller text, as odds are that you’re using a resolution that is less than the actual screen, and the screen’s bits are covering more than one dot on the screen, leading to the blurry viewing.
Enjoy.
But why would it have changed? I hadn’t changed the settings but it still went blurry.