.php Error

Use FTP to replace the functions.php file with a fresh, unedited copy. See Filezilla and FTP Clients « WordPress Codex.

And:

White screens are PHP errors. Try Debug and see what PHP errors you are getting. See https://codex.wordpress.org/WP_DEBUG

Add

define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );

in wp-config.php and the debug.log file will be in wp-content.

Add this line

define( 'WP_DEBUG_DISPLAY', true);

to wp-config.php to log and dump them to the browser.