I installed Microsoft SQL Server 2012 developer edition on my laptop first, then I used the same CD Software to install it on my desk top computer at home. It works fine on my desk top computer, but when I wanted to work with SQL ON MY laptop it does not connect to the server,instead I always get this error message: A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Shared Memory Provider, error: 40 - Could not open a connection to SQL Server) (.Net SqlClient Data Provider.
Try going into SQL Server Configuration Manager. In SQL Server Services, make sure your SQL Server service is running. If it isn't, Start it. You should be able to do this directly from Services.msc as well.