There are any number of uptime, and ping services that you can google for. This can raise the alarm in a timely fashion when your site, or parts of your site go down, and then you decide how to handle those issues.

You may also want to google for DNS failover services, to help you automatically redirect traffic in more catastrophic failure cases. There are offerings from google[1], AWS[2], and others.

[1]: https://cloud.google.com/dns/docs

[2]: http://aws.amazon.com/route53/