On which URL are you trying this code

https://www.zoho.com/

I am not getting any alert on this URL

Delete Popup is appeared when i try to delete a Lead

Ok.. please use explicit wait and wait till the alert is present

I have alredy used explict wait and wait till alert is present. Its not working .

it throws timeout exception

publicvoidAcceptAlert(){

try{

WebDriverWaitwait=newWebDriverWait(driver,29);

   wait.until(ExpectedConditions.alertIsPresent());

   Alertalert=driver.switchTo().alert();

   alert.accept();

   System.out.println("alert was present and accepted");

}

catch(Exceptione){

   System.out.println("alert was not present");

   System.out.print(e);

}

}

Use simple pause - life thread.sleep

Also send your project in attachment

LeadTest.Java 

Ingnore above file and consider attched project