Simple Login Lockdown prevents brute force login attacks/attempts on your WordPress installation.
Simple answer: wait. The lockdown will clear in the time you specified, just visit the site again later.
If you absolutely need to get into your site right now, you can can do one of two things...
1. Fire up your FTP client and rename the
simple-login-lockdown plugin folder
2. Login into your favorite database administration tool (probably PHPMyAdmin) and search for
locked_down_ in the
option_name column of the
wp_options table. Delete the records you find -- they should be "transients".