funtions.php file has suddenly disappeared after error message

Hi, I was attempting to install a custom code for Woocommerce(sale code) in my functions.php and after I clicked 'update' I got the following message: Unable to communicate back with site to check for fatal errors, so the PHP change was reverted. You will need to upload your PHP file change by some other means, such as by using SFTP. After that the entire file went blank. I've tried pasting the functions.php file from my offline backup but I keep getting the same error. Please help! Thanks! -Adam