Some of you are familiar with this Wordpress error, it appears from time to time depending of what plugins you have installed. I've met it earlier when I upgraded an Wordpress package to version 3.5 and the plugins on that particular website triggered this error... and locked me out for a few minutes from the administrator section.
The solution was very simple: I logged in using the ftp, I renamed the plugin directory and I tried to access again the administrator section. Of course, another error appeared: some plugin files were missing. I renamed the plugin directory to the original name and the access was restored.
Usually you can find that directory in your-wordpress-directory/wp-content/plugins/. A lot of people are installing wordpress package in the root of their blog, but that is not a rule, it's just a habit.