My web server was down for about 30 min. The next day (30~ hours from the event) I noticed major drop in traffic from Google. The normal traffic to my site is about 2500 unique/day and after the event it dropped to 400/day. Other than this event nothing happened. Can this be the reason?
Leading up to the event search traffic may have been higher than it is now and not have to do with your server having problems. Just like traffic goes up during the holidays. I guess it would depend on the event if it was locally or nationally promoted. 30 minutes of downtime shouldn't affect your traffic.