Actually, the admin access is failing. It is when i try to login to my admin area, i get this fatal error without loading the admin interface.
I actually changed the file name via FTP as you suggested and luckily the admin interface loaded for me. Then I undid the file name change after navigating to plugins page in the admin interface and now I de-activated the plugin. It then worked perfectly fine.
I have now upgraded to latest WP v2.9
Thanks