Unable to Login to Admin WordPress Fatal Error:Call to undefined Function

I am having trouble logging in to administer my wordpress website. After I enter my username and password I get a screen that displays the following message.

Fatal error: Call to undefined function wp_menu_unfold() in /home/content/35/5868135/html/wp-admin/admin-header.php on line 31

If I open a new window in my browser, I can see that I am logged into the website, and there is the admin menu bar available. The drop down menu bars seem to work but when I select an option on the menu, It reverts to the fatal error message above. The only drop down menu bar that works is the Logout option.

My WordPress website is hosted by Go Daddy. I have looked up the file on the Go Daddy FTP page and downloaded the admin-header.php file to my computer, opened it using notepad, and saw the text: wp_menu_unfold() in the admin-header.php file. I assume I can “fix” the file using notepad and upload it back to the Go Daddy FTP page and it should work, I just don’t know how or what verbiage or info would fix it!

I’ve had this problem since about August 2011, but haven’t felt like attempting to fix it till now. I assume the problem occurred after an automatic update, but I’m not sure about it.