So, I went to update a plugin, then accidentally clicked away while wordpress was still running the update. Yeah. Don’t do that. If you do, FTP to your blog directory and look for a file called .maintenance. Rename it. It should bring your site back up. If it does, then you can delete the renamed .maintenance file.
To be honest, this isn’t an original idea. I first found it here when I borked my wordpress install as described above. Quick google, and I found it. Here’s the link: http://wordpress.org/support/topic/blog-now-stuck-in-maintenance-mod