This is the most frequently asked question. On the Windows platform, the Java support seems to frequently become unstable. The result is that an applet that loaded OK before no longer loads.
If rebooting and/or clearing your browser cache does not solve the problem then read on:
The easiest thing to do is to update the Microsoft Java virtual machine and replace it with the official Sun Java one. This can be done by visiting http://www.java.com/. Don't worry, the update is easy. Click on "free download" in the upper right corner, then follow the instructions on the next page.
For Macintosh, with Mac OS X, both Internet Explorer and Mozilla with the current Java version apparently work OK.
If you're having difficulty with the site's tests and suspect a Java problem, please see this: