Question re: robots.txt. file

In webmaster tools for one of my sites, the robots.txt file says that one url is blocked.

I have no idea what url this would be. I created a sitemap two years ago, but was the robots.txt file automatically created from that? On the sitemap, I recently (about six weeks ago) changed the sitemap to not include tag pages, author pages, things like that, but I never anywhere put in a url to be blocked. And I have no idea where to go to see what this url is.

Anyone reading this can probably tell that I barely know what I'm trying to talk about. I wasn't even sure which section of the forum to put this question in.

My site used to be ranked pretty well in google, but now it's nowhere to be found, although I'm still ranking in yahoo and bing. I did very little backlinking,and all of my content is original. However, google did cache my site just a few days ago. I did the "google fetch" function in webmaster tools for a few of the pages, and they were fetched with no problem.

But this thing about the the robots.txt file that's supposed to have one url blocked bothers me. Before I start a campaign to try to bring my site back, I want to make sure there's nothing on the technical side that is a problem.

Can anyone please educate me on this issue? Thanks so much.