I had this issue due to not enough space on the server. I was able to add space and it immediately worked fine for me. I guess you could try deleting some old files to free up space also.
I had the same issue. My solution was to make more space available on my server to allow for the upgrade to complete. Here is the sequence of events:
1) Automatic update from WP dashboard caused error and completely blocked my access to the site:
“Call to undefined function wp() in /home/DOMAIN/public_html/wp-blog-header.php on line 14”
Tried a few solutions like removing plugins that did not work.
2) Tried to update from Fantastico in cPanel and that failed, but noticed my available space had all but disappeared.
3) Deleted several unused files from my server (also increased quota since I have access to that).
4) Upgrade through Fantastico in cPanel worked like a charm!
Hope this is useful to those who setup WP through Fantastico. Will try to remember to keep an eye on available space! Less invasive that having to do all the manual upgrade steps. Good Luck!
