Enable Dedicated Administrator Connection

Are you attempting to connect from the local server, or a remote machine? Using SQLCMD or SSMS?

Here are a few things to keep in mind when working with the Dedicated Administrator Connection (DAC):

Verify the account you are connecting with is a SysAdmin on the SQL Server. The DAC is only available to members of the SysAdmin server role.

By default, the DAC is only available on the local server. If you are trying to connect from a remote machine, you will need to enable network connections:

EXEC sp_configure 'show advanced options', '1'
RECONFIGURE WITH override

EXEC sp_configure 'remote admin connections', '1'
RECONFIGURE WITH override

If you are using SQLCMD, be sure to use the -A switch to access a DAC. If you are using SSMS, be sure to prepend "ADMIN:" to your server name.

Here is a brief overview from MS

See the following links for information on using the dedicated administrator connection:

http://msdn.microsoft.com/en-us/library/ms189595.aspx

http://www.mssqltips.com/tip.asp?tip=1005