There is a full upgrade process to go through to ensure your project is properly converted, see links below.
Have you checked the event logs to get an idea on what is not working or why? This would be a good place to start unless you can start the upgrade process over with the backups you took before upgrading.
Upgrade docs:
http://devnet.kentico.com/articles/kentico-8-upgrade-is-out
API Changes:
http://devnet.kentico.com/articles/api-changes-in-kentico-8