Have a look at this video from Matt Cutts, head of Google Webspam team explaining how to get your search rankings back after your site has been down.
Video Transcription :-
“Make sure you put your website back up, make sure that its got reliable uptime and make sure that all the pages that were there before are still there. So, there is a tension at Google where when a page goes away may be there is a 404, we don’t know whether it has really gone away, or whether that page will be back. Sometimes there is a server time out, servers are kind of slow, so on one hand you don’t want to keep showing a page which be a bad user experience like it has really gone away, but, on the other hand, it’s very common for the websites to go down for an hour, two hours, a day, may be two days and so you also get the benefit of the doubt so you can revisit those pages and see whether they have gone up.
So, we do have varying time periods where we basically allow if a domain looks like it has gone away but it comes back online then we just say it was a transient error. So, the short and simple advice is, just go ahead and make sure that you put the website back to where it was and hopefully things should recover very quickly.”
Also See:-
Google Introduces 3 New Methods for Easier Management of Site Verifications
Search Queries in Webmasters Tools
Google Tag Manager
Google Adwords Bidding
Multi Channel Funnel in Google Analytics
Seo Secrets
How to rank in Google
Google Disavow Links Tool
Battle of Ranking Continues
Types of Crawl Errors
How to rank in Google
Link Baiting
How to Remove Url’s from Google Index
Free Seo Tools
PageRank Dissipation is Roughly the same between 301 Redirects and Normal Links
Google will continue to support Toolbar PageRank