To solve, go access your site files through FTP or cPanel…In the plugins folder of your blog, delete this plugin folder and may found it running.
An alternative way is to disable all the plugins through your cPanel or FTP program. To do this, rename your plugin folder to something like “_plugin”.
You can find a post on my blog where I faced such problem while upgrading to WP 2.9
https://www.smartbloggerz.com/2009/12/wordpress-2-9-carmen-whats-new-problems-you-could-face/