Pete wrote a blog about expected downtime on your website and protecting your SEO work.

However not all server downtime is expected or planned for in this way and so it is important to know if you site goes down all of a sudden so that you can get this resolved as soon as possible.

If the SEO on your website is working well and you have a blog on your site Google might be visiting your website more often that before you began working on Search Engine Optimisation.  If Google, or the other search engines visit your site while it is down this can impact on your search engine rankings, mainly on the second visit by search engines spiders as if they encounter site downtime on two consecutive visits they will assume that the site has been down throughout this period.

This means it is important to ensure that the website is back up again as soon as possible.  The easiest way to do this is to use one of the free website monitoring tools on the internet, there are a wide range of free tools which will email you when the site goes down and let you know when it is up again, this will mean you can contact your web hosts as soon as you get an email telling you that your website is down, which should ensure that your site is back online again as soon as possible.