the server was not found or was not accessible

Hi
I have installed SQL Server 2008 R2 Express and I have created Instance and Database and Tables
but now I've got a problem  SQL Server doesn't work (does not open) it gives me this 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. (provier: named pipes provider, error: 40 - could not open a connection to sql server) (microsoft sql server, erro: 53)
...