Cannot login to network server in sql server

GO to "Sql Server Configuration Manager"

Select Sql server Services

Check following service are running
- Sql Server(Instance Name)
- Sql Server Browser

Go to "Sql Server Network Configuration"
- Select Protocol for <Instance Name>

 Now check TCP/IP and Named Pipes is Enable.

and make sure Your system Firewall should not block SQL Server port. SQL SERVER port is by Default 1433