There are several methods to find out whether a website to detect downtime, has stopped working
1. using a website monitoring service
2. visiting the website regularly
3. PPC ad networks like Google adsense will not send a notification, however, the web statistics will show a decrease, especially for high traffic websites. The loss of revenues cannot be reversed, hence it will be important to have an independent monitoring mechanism, so that the webmaster will be alerted within an hour, to take corrective action
4. Other ad networks send a notification,which may be stolen due to the high level of corruption in the indian internet sector, and the webmaster has to act if he or she receives the notification.
5. for ecommerce websites there will be a significant decrease in visitors and sales.