I was Upgrading from DNN 4.04 to 5.1.2 community edition
On the first access to the site after downloading the 5.1.2 upgrade files
While it was installing the update I got this error:

Upgrading Config Files  04.07.00 Error

All other lines showed success

When I click on the link at the bottom of the install page to access the portal I got a screen with 2 lines saying "An error has occurred"

The following line was in the address bar on the browser.

http://www.geosolarandwind.com/Default.aspx?tabid=36&error=Object+reference+not+set+to+an+instance+of+an+object.&content=0

When click on return to site, nothing happens....just keeps redisplaying the error.

I can't get to the site to do anything.......any suggestions?

Looks like you didn't used the new web.config file witch is included as release.config (Update) or as web.config(install/source), or you didn't have transfer the settings specially the maschine Key to the new file.  If this is the issue, you have to restor the database and files you have done before you run the upgrade.  Then retry upgrade with make sure you have all diffents on the old web.configfile added to the new one.