Fatal error with general-template.php

I should start with some back story of how I arrived at this problem:

I wanted to change my calendar widget in my theme to show posts by year, rather than by month. After searching through the forums and google indexing, I found that it required a modification to the general-template.php file in the wp-includes folder.

Since I am not a programmer (or know how to use php!) at all, I decided that I would copy over the original document’s contents into a .txt file so that in case I buggered up the general-template.php file, then I could just copy the contents from the .txt file and paste it back into the general-template.php file and have the original working contents back!

I did bugger up the file, and so I pasted the .txt file contents back into the general-template.php file, but then it still did not work.

I then tried to unzip the original file from the zipped wordpress file and then copy that one over to the file stored on my server via ftp, but that also did not work.

This is the error I am getting:

Fatal error: Call to undefined function home_url() in /var/home/steph/sites/www.sciencequeen.com/juiceboxmixes/wp-includes/general-template.php on line 407

I searched this error in the wordpress forums here and I also tried the google site indexing thing again, and still no solution. I sent a message to my friend that installed the wordpress onto my server, since he would likely be able to help, except he has a full-time job and a 5-year-old and has better things to do than listen to me bitch about something he just installed for me a few days ago. ?? (he does me the favour of hosting it – I don’t pay him very much)

I even went so far as to download a php editor to see what this “line 407” business was all about, and all it shows is:

$output = home_url();

I don’t know what it is referring to! The error above to me sounds like it’s referencing something that does not exist, but I don’t know WHY it does not exist, or how to make it “come back”??

Any help or insight would be much appreciated!!

Also, though I have access to the files on my server via ftp, I don’t have access to the programs/control panel etc for where the wordpress was installed through.