How to restore the default vCenter Server alarms

To restore the default vCenter Server alarms, you must remove all existing alarm definitions in vCenter Server and then set the value of alarm.version to 0.
To delete all the existing alarm definitions in the vCenter Server:

•     Log in to the vCenter Server using the vSphere Client.
•     Click the Hosts and Clusters view.
•     Click the Alarms tab.
•     Click Definitions.
•     Note: All custom alarms must be manually recreated after they are removed. Ensure to note the custom alarms before removing.
•     Select all alarms, right-click, and  click Remove.

To restore the default vCenter Server alarms, change the alarm version in vCenter Server:

•     Connect to vCenter Server using the vSphere Client.
•     Click Administration > vCenter Server Settings.
•     Click Advanced Settings.
•     Set alarm.version to 0.
•     Set alarms.upgraded to false
•     Restart the VMware VirtualCenter Server service.

For more information take a look at this VMware article.