It sounds like you may have deleted everything then, including Wordpress, if /blog was where it was stored.
See, the theme, in this case, Atahualpa, is, in a sense, the framework through which we see your site. Wordpress is the engine running the site, or in your case, your blog. The default theme that comes with Wordpress, Kubrick, is just a very simple framework to show your blog to the world. When I was first trying all this out a few months ago, I had 15 themes installed! Wordpress lets you try them all out without losing your data. Your data, your posts, are sitting in a database away from the folders where your blog is.
No need to panic. All of this can be downloaded again, and it's free. Lots of people with no prior experience with this stuff are able to set up Wordpress and then add a theme like Atahualpa, so you probably can too. You seem to have FTP access to your server. You could try it yourself, or ask your webmaster to re-do it, or maybe your ISP support folks would install it for you. After it's all in place, Atahualpa is a breeze to work with, especially if you aren't trying to tweak the bejeezus out of it like I do.
Flynn has given us a very flexible and customizable theme, and it's only because I'm trying to make Wordpress be a content management system for a large site that I need a lot of extra tweaking.
One thing you will need to know, if you try this, is the path to your WP database, because you have to open up a WP file, wp-config-sample.php, and put in your database name and your username/password. Then when you upload the file, you have to rename it to wp-config.php.
If all this sounds too geeky for words, it is.
It's fun, though, once you get into to it a bit. I've probably confused you even more than you were already, but take heart. All of this can be fixed, and because WP and Atahualpa are free, your only costs will be in time and whatever you may have to pay somebody to set it up again.
Go to http://wordpress.org/download/
and download Wordpress again. Read the Handy Guide, and other good stuff on the site. Email or call your ISP and find out the database information, and/or ask them to do all this installation for you, and then I'll bet you can manage things after that.