So first you should not modify any of the core WordPress files, ever. Otherwise, your changes will be lost the next time you upgrade WP.
Instead you want to modify your theme’s function file (if it is your own theme) or make a child theme and modify its functions.php file. (if you are using someone else’s theme)
As for figuring out the error, it will usually appear in your web server’s logs. If you can access those, and show us the error, we should be able to help out.
Assuming you only changed that one file, another option would be to download a fresh copy of WordPress on your computer, grab the wp-includes/functiosn.php file from that fresh copy and replace the one you modified with it. At least then you’ll know for sure you are back to the original file.
But don’t panic, don’t do anything else in code until you’ve figured this out, just to be sure we don’t lose anything. Everything you’ve done is probably still there – just not loading correctly.
Hope this helps!