Hey Akshay, following code snippet would help you in understanding how to restore cookies in new browser window:
public class CookieTest {
    WebDriver driver;
    @Test
    public void login_state_should_be_restored() {
        driver = new FirefoxDriver();
        driver.get("http://www.facebook.com");
        driver.findElement(By.id("email")).sendKeys("test123");
        driver.findElement(By.id("pass")).sendKeys("pass123");
        driver.findElement(By.id("u_0_2")).click();
        Assert.assertTrue(
                driver.findElement(By.id("_2md")).isDisplayed());
        //Before closing the browser, read the cookies
        Set allCookies = driver.manage().getCookies();
        driver.close();
        //open a new browser window
        driver = new FirefoxDriver();
        //restore all cookies from previous session
        for(Cookie cookie : allCookies) {
            driver.manage().addCookie(cookie);
        }
        driver.get("http://www.facebook.com/");
//Login page should not be disaplyed
        Assert.assertTrue(
                driver.findElement(By.id("_2md")).isDisplayed());
        driver.close();
    }
}