SQL 2008 Connection Problem

Everything has been fine in connecting to SQL Server 2008 default and the named instances. But today the default instance cannot be connected with an error:

TITLE: Connect to Server

Cannot connect to XU-PC.


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: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server) (Microsoft SQL Server, Error: 2)

But it is OK to connect to the named instance and SSIS/SSRS/SSAS.

Reason: the default instance service, which has been configured automatically starting, was not started for some reason.