Can't log into SBS 2011 server with accounts that have administrator priviledges

[Mindly]

With accounts that have administrator privileges, can no longer log into the SBS server. Will get error that the user name or password is incorrect. This occurs whether the login attempt is made locally at the server or remotely via Remote Web Access.
Users without administrator privileges can successfully log in via Remote Web Access.
When performing an operation at a workstation that requires administrative privileges such as installing software, the administrator accounts are accepted as sufficient credentials to perform the operation, but the exact same credentials fail when attempting to log into the server.

I no longer have administrative access to the server.
Any ideas as to how to get in?

Thanks