Dedicated Admin Connection or DAC for short will give you a single superuser connection to the DB. DAC can be used on both SSMS and SQLcmd. When using SQLcmd, use the -A switch to get DAC access to the DB server.
Using DAC with SSMS is bit more tricky. First of all, it cannot be done from the object explorer. Open up SSMS and cancel out of the current connection dialog box. You are now presented with SSMS without a connection. Click on ‘New Query‘. In the ‘connect to server‘ dialog box specify admin:ServerName/InstanceName. Generally only the server/instance is specified. Once the ‘admin:’ is inserted at the beginning, DAC is setup. For further reading, click here.