Accidently deleted a line in Footer PHP. How do I undo it?

You could download the theme as .ZIP, then extract theme-functions.php in Nirvana\Includes\ and replace this file with your modified version.

The snippet(s) you’ve probably modified originally looks like this:

/**
 * Site info
 */
function nirvana_site_info() {
	$nirvanas = nirvana_get_theme_options();
	foreach ($nirvanas as $key => $value) { ${"$key"} = $value ; }	?>
	<em style="display:table;margin:0 auto;float:none;text-align:center;padding:7px 0;font-size:13px;">
	<?php _e('Powered by','nirvana')?> <a target="_blank" href="<?php echo 'http://www.cryoutcreations.eu';?>" title="<?php echo 'Nirvana Theme by '. 'Cryout Creations';?>"><?php echo 'Nirvana' ?></a> &
	<a target="_blank" href="<?php echo esc_url('http://wordpress.org/' ); ?>" title="<?php esc_attr_e('Semantic Personal Publishing Platform', 'nirvana'); ?>"> <?php printf(' %s.', 'WordPress' ); ?></a></em>
	<?php } // nirvana_site_info()
add_action('cryout_footer_hook','nirvana_site_info',15);

/**
 * Copyright text
 */
function nirvana_copyright() {
	$nirvanas = nirvana_get_theme_options();
	foreach ($nirvanas as $key => $value) { ${"$key"} = $value ; }
	echo '<div id="site-copyright">'.$nirvana_copyright.'</div>';
} // nirvana_copyright()