This question has been answered by dubbs. See the first response.
Check out the error logs. Both the server error log and the MODX error log (you can view that via the filesystem too, core/cache/logs/error.log).
Also have a look at the developer console of your browser. There's probably a bunch of errors in the console. There might be some failed requests in the Network tabs as well.
Is it just happening for you? If so, you might get some result by clearing your browser cache.
Also have a look at the developer console of your browser. There's probably a bunch of errors in the console. There might be some failed requests in the Network tabs as well.
Only one error in developer console of Mac FF which is:
Timestamp: 02/05/2013 21:39:01
Error: TypeError: can't convert undefined to object
Source File: http://modx.com/assets/components/discuss/themes/modx/js/sh/shCore.js
Line: 20
Resource interpreted as Script but transferred with MIME type text/html: "http://www.example.com/". www.example.com:11 Uncaught SyntaxError: Unexpected token < /:1 Uncaught TypeError: Cannot read property 'prototype' of undefined ext-base.js:7 Ext.apply.extend ext-base.js:7 (anonymous function) index.php:12 Uncaught TypeError: Cannot read property 'prototype' of undefined ext-base.js:7 Ext.apply.extend ext-base.js:7 (anonymous function) index.php:1 Uncaught TypeError: undefined is not a function ext-all.js:7 create ext-all.js:7 Ext.extend.load modx.js:74 (anonymous function) www.example.com:20 (anonymous function) ext-all.js:7 b ext-all.js:7 Uncaught TypeError: undefined is not a function ext-all.js:7 create ext-all.js:7 Ext.extend.load modx.js:74 (anonymous function) www.example.com:32 (anonymous function) ext-all.js:7 b ext-all.js:7 Uncaught TypeError: undefined is not a function ext-all.js:7 create ext-all.js:7 Ext.extend.load modx.js:74 (anonymous function) www.example.com:40 (anonymous function) ext-all.js:7 b
[2013-05-02 21:36:10] (ERROR @ /index.php) Could not get table name for class: modFileMediaSource
That error in the log is nothing to worry about.
Those JS errors indicate some failed script loading. It sounds like it is loading the site homepage instead of a js file, which could come from the config JS not loading properly.