Check once that your sqlserver is started
To start the service follow the steps bellow
Control Panel-->Administrator Tools-->Services--->(Select)Sql Server[Your server whic you use to connect] --on Left top click on Start
If the Sqlserver has the Sql Server Authentication Always use trusted connection with the related username/password
hope this will help..
You can have a better idea about the connection string just follow my blog post hereConnection string properties