Windows Server Standard suddenly will not accept Domain Administrator credentials for console login. Username or password incorrect

I have a Windows Server 2008 standard 64bit server setup and has been running for a month or two and suddenly it will not accept the known good Domain Administrator credentials for a RDT or local login. I have tried

domain\administrator
administrator@domain

I know the password is correct and has been working, and to my knowledge no changes have been made to the local or domain policies.
Automatic updates haver been applied as they are issued.

I can log on as another domain account without an issue but not the domain Administrator account, so domain communication is working.
Firewall is off
complex password local policy is disabled

ANy Ideas?