Reboot the server in safe mode. You should be able to logon with the default administrator account, even if you disabled it. Then re-enable it and reboot to normal mode.
Enabling safe mode in 2012:
boot using a Windows Server install DVD or USB, and select REPAIR YOUR COMPUTER, then Troubleshooting, then Command Prompt
then:
bcdedit /set {bootmgr} displaybootmenu yes
bcdedit /set {bootmgr} timeout 10
Reboot your machine, pressing F8 and SAFE MODE