problem with upgrade

I tried uploading the latest version of WordPress and now I’m getting “fatal error” messages. I can’t seem to get my blog back! Can someone help me? This is the message I get:

Fatal error: Call to undefined function is_admin() in /home/homescj8/public_html/wp-includes/vars.php on line 16

Help! I’ve posted on other forums with no results and my server won’t help me either. Yikes!