Are you certain it was because you were down for a few days? Or are you guessing?
It's unusual for Yahoo to remove a site simply because it was down a few days. There are plenty of sites in the directory that are so old they don't exist anymore. It's why people buy up expired domains that have Yahoo listings.
Was it a paid placement, or a free listing? If it was paid, you should be able to reply to the email that told you you were listed and ask what happened. If it was free, you can try emailing them, but I doubt you'll get an answer. Just submit it again.