Be sure not to confuse Java with JavaScript. Java is an applet that operates on your system like Flash and other applications.
JavaScript is a scripting language that is contained on the server as coding. When a visitor loads the page in his browser, the JavaScript gives commands to the browser for various functions, everything from opening smaller windows, to calling up CSS elements, such as style sheets.
Sometimes, if the Page is loading improperly, the Scripting isn't "read" by the browser properly and chaos ensues.
IN this modern age, a webpage is no longer a solitary document with HTML content and some includes, it is a shell that is populated from several sources within a database, that becomes assembled in the browser. So if one script is slow to respond from the database, the page fails.
Usually, refreshing the page will solve this problem and in some extreme cases, closing the browser and reloading will fix it.