If you need a website monitoring tool, Pingdom is the way to go.
It offers a free (one website check) solution to start monitoring your website:

Register at https://www.pingdom.com/free

It has an easy user interface to setup your required monitoring settings.
Every time your website goes down, you’ll be notified by Pingdom (by e-mail).
Once the site is running again, the alert will be closed and you’ll get notified.
Pingdom pings from several locations so you’ll be sure your website is available all over the world.

Once in a month, you’ll receive an overview of the uptime of your site.

Nice tool to use!