My blog went down for almost 2-3 days due to some problem with the domain name nameservers and once it was resolved i noticed that the traffic went down markedly! remained almost 2% of the usual traffic on the first day.
However, gradually there was increase in traffic, first 5% then 10% and so on, and finally after 2 weeks, the traffic reached back to almost the same level of visitors per day via google search as they were before the site went down.
However the down time does have the impact on your site markedly. It not only reduces your earning by reducing the visits for some days or weeks, but also compromises the repute of your website/blog.
So make sure you know before hand if there is going to be any issue related to your domain name or web hosting server.