If you get an error message like this:
Fatal error: Call to a member function getDbVersion() on a non-object in /var/www/magento/app/code/core/Mage/Core/Model/Resource/Setup.php on line 136
Then it looks like your app/etc/config.xml file has been corrupted somehow.
Restore this file from a backup or otherwise replace it and hopefully you should be back in business.
