Upgrading blog to wordpress 3.5 or higher and getting this error? well well well I spent almost half day behind solving this issue. I ran automatic upate and site went down nothing coming other than Fatal error: Call to undefined function get_current_blog_id() from user.php file.
This is caused because wordpress 3.5 has come with database level changes.
First and foremost thing I’ve re-run update because in the past also I have faced some issues which made me remember that if your blog doesn’t show anything and goes blank or with errors. Try to run update again
http:///wp-admin/update-core.php?action=do-core-upgrade
and make sure it finish successfully.
This error generally comes when your current theme is not supported by wordpress or still something is remaining to update in your wordpress so first reset your current theme to default.if you can’t access Dashboard you can update it via database (Change WordPress Theme From Database phpMyAdmin) and check if your blog is working.
Still not..??
Try to run update again
http:///wp-admin/update-core.php?action=do-core-upgrade
I know it’ll post you the error page but then also keep trying to re-run update till it finish sucessfully.
Now it may be possible that you have not de-Activate plugins/themes while updating your blog. so give permission to your wp-content folder to 777 for time being and again re-run update and try to open your admin Dashoboard. If you are running on windows hosting make sure your IIS is having rights to write in your wordpress directory. specifically if you are having hosted space on godaddy, just be calm.. be patient.. allow write permission to all directories of site and re-run update and wait for a while.. it takes more than 10 minutes sometimes to update wordpress. Close everything.. restart the machine.. Have a coffee break and then check your site. it will be running fine for sure.