In PRTG, there is an inheritance mechanism. You can define notification triggers for objects higher in the hierarchy and inherit them to all objects below (further down in the tree).
For example, you can define a notification trigger for a "down" status in the Root group's settings. To do so, go to the settings of this group, choose the Notifications tab, and add a new trigger. By default, this trigger will then be inherited to all other objects, so you will get a notification message whenever a sensor goes into a "down" status. (However, you can disable inheritance for single objects manually.)
For more information about notifications, please see manual section Notifications.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.