Drupal can be very memory intensive CMS, especially if you have lots of menus, blocks and modules. If you exhaust the allowed memory size, you would get an error such as the following
Fatal error: Allowed memory size of 16777216 bytes exhausted (tried to allocate 41 bytes) in somefile
To fix this problem, you need to allocate more memory.
1. Open you php.ini file If you don't know where this file is, run locate php.ini. Mine is located at /etc/php.ini. Make sure you back up this file before editing it.