The error message means that you were not able to reach the SQL Server instance you have specified. This could be due to a number of reasons:
- SQL Server is not running.
- There is no SQL Server on the machine you have specified.
- You have misspelled the server and / or the instance name.
- SQL Server is running is a named instance, but you did not specify an instance name.
- There can be firewalls in the way.
And the list goes on. Since I don't know anything about your environment or what you are trying to connect to, I cannot be more specific.