Unrecognized configuration  section 'connectionStrings'

Description: An error  occurred during the processing of a configuration file required to service this request.

Cause: Possible cause for this error is application is still being running under ASP.NET 1.x

Solution: Set IIS to default to ASP.NET 2.0. By default, it stays at 1.x, and each new site needs to be set to 2.0.
In the IIS >View the properties for your website/application in IIS > ASP.NET tab > Finally Select 2.0.x to use the latest version.