Matt Cutts, head of Google search, has recently stated that websites that are disabled or inactive for typically more than 24 hours will be removed from Google search.
This makes sense as it maintains a good user experience. Nobody wants to do a Google search and get a 404 Page Not Found when clicking a link.
Google recognises that there will be instances where websites are temporarily down, e.g. due to a server outage. For short term downtime, typically less than 24 hours, Cutts has suggested that rankings will remain the same. This implies that sites that are down (for whatever reason) for over 24 hours may lose ranking position.
The Googlebot spider searches websites typically several times per day and webmasters are immediately notified (Google alerts) of any issues or outages such as site down. These alerts should be actioned as soon as possible.
Somerset Web Services, SEO specialists in Taunton, constantly monitor and action all alerts we receive from Google on behalf of our customers with support agreements.
Please note, businesses choosing to disable a website for over one day are advised to consider the long term impact on search engine rankings. This would also apply to any pages that are removed or hidden.