Don't host your site with a host that is down a lot

While search engines won't deliberately discriminate against a site that is down, if they can't get to your URL because it's down, they can't index it. And if your site is down several times when the spider tries to access it, it could be flagged as gone, and then the spider won't come at all. Find out from your hosting provider what their uptime rates are and what they guarantee. Less than 97-98% uptime is bad.