To fix your site, you should get another
As such, you can get that from your most recent and operational backup and upload it unto the location of the mistakenly deleted one.
In case you don't have a backup, your last option will be to download a fresh copy of your theme; once that done, extract and upload the
functions.php file it contains unto the location of the one you've mistakenly deleted from your server.
In the later case (should you have edited directly your
functions.php - which is not a recommended practice), you cannot recover any previous modifications performed on the file.
Should you consider editing the Twenty Sixteen WordPress Theme or any other one from the WordPress Repository or any other source, I will recommend you create a Child Theme of the theme you intended to use for that purpose, rather than editing the actual theme directly.