It may have happened to you. Your website goes down and you are scared for the outcome that it will have on your website’s search engine results. Google have just released a Webmaster Help video where Matt Cutts answers the question: If my site goes down for a day, does that affect my rankings?
To summarise, if your website is down any longer than a day, then it could have negative effects on your rankings (with some allowances). But if your website is only down for around a day, then you should be alright. Matt Cutts also adds that you might want to look around the Google Webmaster forum to see others’ experiences. He notes that there was recently a day where Googlebot itself was having trouble fetching pages. It usually has “pretty good reliability,” though, he says.