If you´ve done an upgrade of owncloud from version 7.0.4 to 8.0.2 and your upgrade failed, have a look at your web server error logs. Errors which looks like
PHP Fatal error: Class OCA\Contacts\Share\Addressbook contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (OCP\Share_Backend::isShareTypeAllowed) in /var/www/owncloud/apps/contacts/lib/share/addressbook.php on line 130
Furthermore you´ve to download both apps from the owncloud app store for the version 8. Now replace your downloaded files with the installed ones. Path: /owncloud/apps/. For sure it´s recommended to deactivate both apps before upgrading owncloud.