I'm without a domain host at the moment, but apparently my previous hosts won't be open for many hours for me to ask them to release my domain, and in the meantime my website, which was aimed at rescuing a bunch of street cats is completely shut down, and I'd love to get a message up on it that it will be up & running again soon. Can I do this??

You need to set your Domain to point to somewhere else a free site or anything. You own your Domain (i hope) which means you should look at transferring it to someone else like Godaddy http://www.godaddy.com they will transfer your domain onto there servers. Hurry before it is lost.