by Luis de Vasconcelos.
Go to http://yoursite.com/admin/settings.php?section=debugging and turn the "Debug messages" level up to Developer. See http://docs.moodle.org/26/en/Debugging
Then go back to your front page. You should get an error message (in Moodle or your PHP error log, depending on what you select under messagesdebugdisplay on that settings page) describing your problem.
Post that error message here...