Since this is typically your only domain controller this can be very disconcerting.
The SBS server will not allow domain admin logon
At boot up black screen you see a warning similar to:
Safe Mode: booting in directory services restore mode
Multiple restarts do not make the server boot normally
At the logon screen change user to:
Enter in your directory services restore password.
Note: If you have a local domain administrator with a logon of “administrator” it may not be the same password.
With the correct directory services restore password entered you should be able to logon to safe mode
go to Start> Run and type in MSConfig
Select the boot tab and change the startup from “safe mode” to “normal” save and exit.
Perform a system restart
The boot and logon process may take longer than normal but be patient your logon troubles may be behind you.
After a successful normal logon process I would recommend checking all services and make sure all it good then restart again just to make sure the issue does not return.
Credit- Charles Mix