get connection string form session to webservice

Hi,

I need to take the connection string according to the userId from the session variable.

According to the user the connection string will differ.So please help me

 [WebMethod]
    [ScriptMethod(UseHttpGet = true, ResponseFormat = ResponseFormat.Json)]
    public string GetData()
    {
        string constr = @"";

        DataTable dt = new DataTable();
        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand("select Month,Predicted,Actual  from _Data where uId= ", con))
            {

                SqlDataAdapter da = new SqlDataAdapter(cmd);
                da.Fill(dt);

                return GetJson(dt);
                //return ConvertTableToJSON(dt);
            }
        }
    }