I've implemented the script using Python and selenium to click on the ads. But now this script is not working.Unable to find element on the page.Please help me to correct the script. Thank you!

from selenium import webdriver
import time
browser=webdriver.Firefox()
browser.get('http://rutracker.ignn.ru')
browser.switch_to_frame(browser.find_element_by_xpath('//iframe[starts-with(@class,"tblock_")]'))
browser.find_element_by_xpath("//a[contains(@href,'tmozs.com')]").click()
time.sleep(10)
browser.quit()

You can try to wait some time until required iframe appears in DOM before switch to it:

from selenium.webdriver.support import expected_conditions as EC
from selenium.webdriver.support.ui import WebDriverWait as waitwait(browser, 10).until(EC.frame_to_be_available_and_switch_to_it(browser.find_element_by_xpath('//iframe[contains(@src, "//tmozs.com/")]')))