Before you read my suggestion did you try clearing the event log and site log. Did this help....I would try to do that first before moving on. I find that clearing these logs first help to make any site upgrades go that much smoother. In fact I always empty recycle bin, truncate event log, truncate site log before I upgrade a site. It really makes a difference.
Just a shot, but I sometimes get this error when a skin or container has been removed. Try manually copying your skins/containers back to your site from your backup, or anywhere you have a copy of the orginal skins/containers that you used.
If you used customized skins and can't restore it, copy the default skin/container folders from the DNN install over to the site and access the DB and change the skin setting via the DB in the Host Settings, and then goto your TABS table and MODULE TABS table and set all the skin src and module container src to NULL.
Then recycle the app pool that the site is using and try to access the site.
And has a last resort, you could always copy the DNN core (INSTALL VERSION) files over your portal...make sure to backup your web.config. Just make sure the install version your copying is the same version of your live portal or it could get real messy.
Hope this helps.
Jerry