According to http://analytics.blogspot.com/2011/05/issue-affecting-analytics-data-for.html there are some problems with custom reports & advanced segments for April. I was already wondering with my colleagues, why our search traffic dropped to zero. So, hopefully this is only a bug with the data, Google is already trying to fix it. Let's see when the GA dashboard at http://www.google.com/analytics/status is showing all green again =)