In most cases this problem is caused by custom third-party JavaScript codes that conflict with the software JavaScript code. I suggest that you use FireBug to find out if there is a java-script error. You can find the information on how to use FireBug - the add-on for Firefox - here.
From now on we have added the possibility to turn off JavaScript on the Index page. This allows you to remove inappropriate code.
How to use it:
http://[yoursite_URL]/index/customize