If, after apparently a successful installation of the SUPEE-5994 patch, you get an error message along the lines of
Class ‘Mage_Install_Controller_Router_Install’ not found
Then here is a suggestion that may help.
A common reason for this failure is that the old codebase is still the version that has been compiled. Here is how to clear your compilation files via the command line: $ php -f shell/compiler.php -- disable $ php -f shell/compiler.php -- clear $ php -f shell/compiler.php -- compile $ php -f shell/compiler.php -- enable This can be run via SSH. The shell folder is in your Magento site root.
I also had a similar problem where the admin was blank. However that turned out to be a file in a module that was overriding one of the core files. When updating Magento with the security patches you need to check the modified files list (/app/etc/applied.patches.list) and check if you have any duplicates of those core files in your own project/namespace as overrides. You may well have to manually add the modifications in to your copy. Alternatively if you have just taken a blanket copy of a core file and are extending the original, just leave in the methods and properties you have added or modified. Remove everything else as they will still be active in the original. Take backups though, just in case, or employ your source/version control software of choice. Backup, backup and backup again.
I also posted this answer on the magento stack overflow website. There are some other useful suggestions there , added by other contributors, if this suggestion fails