I would need specific server information to see what you have done but I would assume you made a mistake editing config files. I would suggest replace your config files with the old ones.
The documentation on upgrade is under the documentations folder of the downloaded code and you never replace your config files. In fact upgrading from 7 series to 8 series you simply want to as our release announcement on PP8 states you want to remove the stylesheets and templates folders under your gallery as those have changed drastically
Then you simply upload everything from the download except the 2 config files which you leave the old ones in place and then you would run the upgrade script select the 2 upgrades you need to run then move on to the vb4 plugins portion of the upgrade then you should be operation.