This was a curious and frustrating problem to fix. After updating the local dev copy of a a magento site, and changing a few local folder permissions the admin area stopped working, giving instead an error message of: ERROR_CONNECTION_RESET.
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.