That may cause two common reasons:

• PHP error occured
• included protected file

First, go to Administrator -> Log and look if there are some errors in it. If there are, they may help you fix them.

If log didn't show any errors, try to look at page source. Some times error occures in such places, where they can't be displayed by the browser, but is clearly visible in page source.

By default system is configured to show all errors and warnings. Therefore, if page source is also empty, you probably have included protected file (see bellow).

CMS security requires all PHP files to have security line that protects files to by accessed directly. Check if you have included file from front-end that should be included only from back-end or vice versa.