How to re-enable content notifications for Notify?
Reading your comment on my question above, I guess you're using Actions &amp; Triggers in Drupal 6.

To see which actions you have defined or to define new actions, go to Administer > Site Building > Actions. There e.g. you can define an action for sending an email. From your question I think you already have done that (in the past). 

To assign this action to an event, you have to go to Administer > Site Building > Triggers. There you can choose (under Content) to assign an action to an event, such as published or updated content.