Forgetting the administrator password for your Windows Server 2012 R2 can be a frustrating experience. However, there are several methods you can try to recover the password and regain access to your server:

1. Use another administrator account: If you have another administrator account on the server, you can log in with that account and change the password for the forgotten administrator account.
2. Reset password with installation disk: Boot the server from the Windows Server 2012 R2 installation disk, select 'Repair your computer,' open a command prompt, and then replace the sethc.exe file with cmd.exe. After that, restart the server, press Shift five times to open a Command Prompt with administrative privileges, and reset the password using the 'net user' command.
3. Use a password reset tool: There are third-party tools available that can help you reset the administrator password for Windows Server 2012 R2. Tools like Ophcrack or Offline NT Password & Registry Editor can be used for this purpose.

It is crucial to regularly update and securely store your passwords to avoid such situations in the future. Implementing a robust password management system can help prevent password loss and unauthorized access.