How can I fix a monitor that suddenly got blurry?

Check that all the wires are connected properly, try re-plugging every connection. If that fails, try a different cable and a different computer - that should give you some idea of where the problem is (screen DVI port, DVI cable, or computer DVI port).

From the information you provided, it seems unlikely that it's a problem with the computer's software or the screen's electronics.

If you boot to a Linux LiveCD like Ubuntu or Knoppix, is the display crisp? If so, it's probably a software problem within windows. if not, it's probably a hardware issue.