The admin page has a lot of extension points that modules extend. If one of those modules fails to run properly, then the administration page only loads half way.
A common error is this:
This is usually caused by classloading issues under the hood of the module. You can see the specific error by doing a "View Source" on the page or looking in the tomcat error logs.
Another common fix for this is to delete the content's of tomcat's "work" directory in order to clear tomcat's cache. See the tomcat home dir on the machine for this folder. Then restart tomcat.