Ok if you see this Don’t Panic it is not the end of the world just a few minutes of yout time. If you found me first I suggest heading over to https://www.techbeast.net/2012/06/13/wordpress-3-4-fatal-error-upgrading-database/comment-page-1/#comment-36094 where Gavin has written a great article on a fix for this issue.
On further testing at my end there is something he did not add to the article. That if you want your plugin back you will need to reinstall it after everything has been upgraded. You can also do the alternative on other sites after fixing this mess and that is make sure you update your plugins first before upgrading to WordPress 3.4 this will stop problems with updates on them.