If you forget your keyword, you can always browse your database using phpMyAdmin to view the tables relating to the jSecure plugin.  Click on jos_plugins and then browse - this will allow you to look at all the settings for your plugins.  You will notice that under the row titled System - jSecure Authentication, the "Params" column will state:

Key=

This is the keyword to access your admin section of the site!  So, to browse there direct your browser at www.yoursite.com/administrator/?keyword :)

Alternatively you can delete the plugin manually (via ftp) and remove this row in the database, and your site should then return to normal without the jSecure redirection.