In late January Apple began pushing out new XProtect definitions that blacklist older versions of the Java 6 web plugin from working with OS X 10.6, and older versions of the Java 7 web plugin from working in OS X 10.7 and 10.8. Apple does not provide an interface for XProtect, the ability to turn it off, or even monitor when the definitions are updated. When this Java blacklist was initially pushed out it caused a lot of confusion in the Mac community, but things have settled down now thanks to some new Java updates that have been released.
If you are running OS X 10.7.5 or 10.8.2, and you find that your Java web plugin has stopped working, update Java using the Java system preference pane.
If you are running OS X 10.6.8, and you find that your Java web plugin has stopped working, use Software Update to download and apply the latest “Java for Mac OS X 10.6 Update”.