I hope you didn't update your quicktime to 7.0.4 or else that's probably your problem. Go to the Mac OSX sections of the forum & have a look at the Stickies at the top of the section. There's instructions there about what to do about (1) QuickTime, & (2) if necessary, reinstalling Java.

Which OSX version do you have? It helps to know since Jaguar, Tiger & panther, etc. all have differences & run different versions of Java, etc.