Connections via the DAC don't fire login triggers. From management studio, open a new query and specift admin: before the server name (admin:MyServerName) and use windows authentication. You must be sysadmin.

If remote DAC hasn't been enabled (the default), you will have to do that from a querying tool on the server itself. If the server doesn't have management studio, you can use SQLCMD. Specify the -A switch

Once in, you can disable the trigger.

DISABLE TRIGGER MyBrokenLoginTrigger ON ALL SERVER

Be very, very careful when writing login triggers. If, for any reason, the trigger fires an error of Sev 16 or higher (object does not exist, database not found, permission denied) the trigger fails and rolls back the login.

I've had a couple panicked, late-night phone calls because of these.