Broken Website

My website is iamzang.com

I wanted to update to the latest WordPress 3.8.1. Whenever I clicked on the ticker message saying “Update Now” I get a blank page.

So I contacted my web host and asked help. And this is what they replied:

“Each time you try to upgrade your WordPress the following error is dropped to /home/iamzangc/public_html/wp-admin/network/error_log file:

[02-Feb-2014 07:22:14] PHP Fatal error: Call to undefined function wp_get_current_user() in /home/iamzangc/public_html/wp-includes/capabilities.php on line 1203

It appears that your WordPress installation is broken. We can recommend you to contact WordPress developers for further assistance on the matter, as we do not provide support for any third-party scripts of our customers.”

So I tried following the instruction on manual update as stated in the WordPress site. And now I can’t open my site. I followed the instruction but I don’t know what went wrong. Now I can no longer visit my website.

What should I do now? Can someone at WordPress please fix this. I have a back-up of my website before doing the update. I need this done urgently.

Thanks