Recovering the entire AD forest involves restoring every DC in the forest. You will have to recover them either from a trusted backup, or by reinstalling AD using the Active Directory Installation Wizard. When you recover the forest, you effectively recover each and every domain in the forest back to the time at which the last trusted backup was performed. By doing this, you will lose any additions to the AD since that backup was taken, such as:
All objects (users, computers, and so on) that were added since the last trusted backup.
All updates made to existing objects since the last trusted backup.
Additionally, if we assume that all of the DCs in the forest no longer function correctly, any software applications that were running on the DCs will need to be reinstalled and configured, once the forest is recovered.
If there has been a corruption in the AD database and it was replicated out to the entire AD infrastructure, you cannot use any of the previously-described ation...