system page 'node_view' disappeared

Just accessed my website today and saw this message:

warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'page_manager_node_view' was given in /home4/eliel/psicodelia.org/includes/menu.inc on line 350.
warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'page_manager_node_view' was given in /home4/eliel/psicodelia.org/includes/menu.inc on line 350.

After that, when I checked the Pages (admin/build/pages) noticed that the system page

(node_view | Node template | /node/%node | In code)

had simply desappeared, and now I can't created a new one.