Redirect users to a maintenance.html file unless they are coming from your IP address. WordPress

mainenance_mode_on.php

/*
	Lock site down to just our IP
  	Create a file /maintenance.html that explains that maintenance mode is on.
	Update your IP address in the line below.
	Add this code to your active theme's functions.php or a custom plugin.
 */
function mainenance_mode_on() {
	if($_SERVER['REMOTE_ADDR'] == 'YOURIPADDRESSHERE')  //find your IP and edit here
		return;

	wp_redirect('/maintenance.html');
	exit;
}
add_action('init', 'mainenance_mode_on');