There are a lot of services offering to monitor the uptime of your website.  Usually services like this cost around $5-$10 a month.  Here’s a little trick that enables Google Documents to monitor your site at intervals of your choice.

First sign in to your Google account and click this link to create a spreadsheet document.  Next edit cells E3 and E5 with your website and email address to send reports of downtime to.  Go to Tools -> Script Editor -> Resources -> Current Script Triggers and then set a time driven trigger.  Personally I set mine to check every minute.  Once saved it will ask for authorization, you just need to click yes.

Google will now send you an email every time your website is down as well as when it is back up.  A detailed report of the server error will also be saved into the spreadsheet you created to tell you exactly what went wrong.