When you use 'MyMachine\MSSQLSERVER', you are trying to connect to a named instance. You have to use MyMachine onlyto connect to default instance.

>When installing SQL Server, you have a choice to install either:
>
>- a "default" instance, which nominally is called "MSSQLSERVER"
>- or, a "named" instance which can be called anything else
>
>If you use a named instance, you have to install the SQL Server Browser service, which I gather acts as a sort of DNS so that client computers can find the named instance on the network. The SQL Browser service does not need to be installed if the only instance on a given machine is the default instance.
>
>Suppose a machine is named MyMachine:
>
>Connect to Default instance, SQL Browser service not present
>- You can just use the machine name MyMachine as the instance name in connection strings -> Works
>
>Connect to a Named instance "MyInstance", SQL Browser service present
>- You use an instance name of MyMachine\MyInstance in connection strings -> Works
>
>Connect to Default instance, SQL Browser service not present
>- Attempt to use instance name i.e. MyMachine\MSSQLSERVER -> Fails
>
>My question is this: if, with just the default instance installed on a given machine, if the (not technically required) SQL Browser service is installed, is it possible to connect to the default instance using MyMachine\MyInstance, or is it still necessary to use just the machine name MyMachine?
>
>i.e. Connect to Default instance, SQL Browser service present
>- Attempt to use instance name MyMachine\MSSQLSERVER -> ???
>
>All of my dev instances are named instances, I don't have a default instance installation to test. Does anyone know or can test the above scenario?