Disaster may happen anything! To recover from AD Disaster, you need to have two important things:
Backup of each NC is required in order to recovery failed domain controllers. The backup contains the NTDS.DIT which, in turn, contains the backup of all the NCs (Domain, Configuration, Global Catalog, Schema, and DNS Application Partitions), DNS Domain Data, System Files, etc.
To recover from a failed domain controller, you log on to DSRM Mode and then run Ntbackup.exe to restore the backup.