messed up with function.php

Whatever changes you made in functions.php, you’ll need to revert them.

Log into your site/server using FTP and download functions.php. Then undo/revert the last changes you made in it, save the file and upload it back.

If you edited post_thumbnails_divi.php as well, do the same for that file.

As added advice, never change theme files directly, but look into using a Child theme instead. That way it’s much easier to fix issues such as these.

https://codex.wordpress.org/Child_Themes

Yes, you’ll need FTP access to the server, so you can download/upload the files from/to the server. You’ll need to download a file to your computer, edit it, and then upload it back to the server.

This is not the process of logging into your WordPress admin.

If you don’t know the FTP servername, user and password, your webhost should be able to help you with that.

that is what the Codex says about FTP – http://codex.wordpress.org/FTP_Clients