This is one of the main reasons we NEVER edit core files. For the love of god.. please use a child theme!!
What you need to do is get a fresh copy of your theme's
functions.php file. Then, you will need to use either an FTP account, or your hosts CPanel to gain access to your file directories.
Lastly.. you will want to navigate to where your theme files are located... and upload the new copy of the functions file... replacing the current one.
EDIT: You might want to make a backup of the "messed-up" functions file.. so that you have a reference of the modifications you made. Then, create a child theme, create a custom
functions.php file.. and work from there.