Downtime is a bad thing. When the spiders attempt to index your site, they will receive a error. Google does not want to send people to sites that won't load so it will decrease your ranking, or even de-list your site.
Now, I don't think this will happen in 6 hours. Google, even with their vast infrastructure has also experienced outages. If the downtime goes on too long, or starts happening more often, then you could have some serious raking problems.
Always make sure you have backups of your site! If your host provides daily offsite backups, that's great, but don't rely on this alone. Have your own backups also! That way, if something goes wrong and you decide to switch providers, you can be back online quickly.