I have upgraded plugins on my blog and when I have tried to open my blog on next day, I am receiving this error message:
Fatal error: Cannot redeclare security_update()
-> DEACTIVATE all the plugins and have a check with your blog..and ACTIVATE plugin on one by one basis so that you can diagnose the issue with the exact plugin. When upgrading a plugin
-> Please take a complete backup of your blog database and install the wordpress again. After that restore the data from backed up database. In this case if any bad files in side the wordpress files then it can be removed.