Wordpress upgrade via fantastico problem

[Gilbert Grace]

As advised I checked fantastico about auto upgrading the wordpress and it showed which version I had installed 2.6.2 and the version it would upgrade to 2.8.6 so I hit the upgrade button and everything backed up ok and I was shown a hotlink to my website. I clicked on the url and got this message,

Fatal error: Call to undefined function wp() in /home/gilbertg/public_html/wp-blog-header.php on line 14

Is this repairable. What to do. I am not conversant with these sites. So no matter how simple they claim they are to use it is all new ground.

Help would be appreciated.

Gilbert