In Moodle,  why is a particular page blank or incomplete?

Does this happen when trying to open a PDF resource? 
 This issue may occur when a user tries to open a PDF resource due to an incompatibility between Moodle and the Adobe Reader plugin on some of the most recent browsers. Generally speaking, refreshing the window (clicking the Refresh/Reload button, or pressing F5) will eventually bring up the PDF , though sometimes it takes two or three tries. This quickly becomes annoying to the user. To resolve the problem, select the “Force download” option in the resource settings when posting a PDF . 
 We also recommend updating your browser and Adobe Reader plugin to the latest version in order to optimize compatibility. 
 If this is not a PDF display issue, it may indicate that an error has occurred when Moodle is trying to display a page. Please let you system administrator know about the problem. 
 (Details for system admins: Depending on the server’s configuration, PHP might hide error messages. If this is the case, when an error in a PHP script occurs, PHP might choose to not display anything, resulting in a blank page. 
 Usually, PHP should not display error messages in a web page for security reasons. However, there are exceptions, e.g. when you are debugging a web page. 
 To globally enable error messag display, put these in your php.ini file: display_errors = On error_reporting = E_ERROR 
 To enable that for a single PHP script, put these at the beginning of the script: ini_set(‘display_errors’, 1); error_reporting(E_ERROR); 
 See the Moodle FAQ for details.)