Michelle,
I've seen this happen occasionally before with the archives. My hunch
is that the server goes through the database and call the messages
into its search engine at some prescribed point (once a day, for
instance) and every once in a while an error causes zero messages to
load making the search engine unable to find anything. I've seen this
happen since I started using the archives about a year ago and it
usually comes back in 24 hours or so. I know I was able to
successfully search the archives early Monday morning so hopefully it
will indeed be a short term problem. Perhaps if one of the
administrators sees this they can ask the UW support to look into it.

Google does a pretty good job of caching Tweeters archives as well.
Here is a query for your email name for instance:
https://www.google.com/webhp#hl=en&tbo=d&q=+site:mailman1.u.washington.edu+tweeters+yelmbackyard

Josh Adams
Lynnwood, WA
http://www.flickr.com/photos/beastofexmoor/