Object reference not set to an instance of an object.

I have developed a project in .Netfromework 3.5., I used linQ also... Its working in my local machine.

Now my hosting server have .net fromwork2.0 and 4.0, I have pushed under 4.0 version, but i run that in live, its getting error like.,

Object reference not set to an instance of an object. 

public static string CONNECTION_STRING = ConfigurationManager.ConnectionStrings["GKAstroConStr"].ConnectionString;

but i gave the same key["GKAstroConStr"] in my webconfig file. How can i solve this error.