Sometimes sites get broken after an upgrade due to a bad theme or plugins. Renaming the plugins directory will usually fix the problem and renaming the theme will sometimes help, however, sometimes the theme is just too problematic and it needs to be switched back to the default theme.
Go into the cPanel, look for the Databases section and click the phpmyadmin link. (note you may need to setup a password using the “Reset MySQL Password” icon before you can log in.)
Select your WordPress database. (If you aren’t sure which database to select, check your wp-config.php file in the root location of your WordPress installation)
Select the ‘wp_options’ table (if you have another table prefix ‘tableprefix_options’)
From the header bar choose the “Browse” tab.
Select page 2 from the tables footer:
Now you’ll need to edit ( ) the ‘template’ and ‘stylesheet’ rows and put in the word “twentyten”, or put in any other theme you want there. Press “Go” when you are done.