reset form input field

Hi

I’ve using this tutorial to create a custom admin menu where the administrator can write a message that will display on the frontpage. But if there is no message, the frontpage output should be hidden and the input field should be cleared.

My question is; how do I create a clear/reset button that hides the css class on the frontpage?

Thanks for reading this far 😀
//MadsRH

This is the plugin:

/* Runs when plugin is activated */
register_activation_hook(__FILE__,'hello_world_install'); 

/* Runs on plugin deactivation*/
register_deactivation_hook( __FILE__, 'hello_world_remove' );

function hello_world_install() {
/* Creates new database field */
add_option("vigtigt_data", 'Default', '', 'yes');
}

function hello_world_remove() {
/* Deletes the database field */
delete_option('vigtigt_data');
}

/*———————————————————————*/
if ( is_admin() ){

/* Call the html code */
add_action('admin_menu', 'hello_world_admin_menu');

function hello_world_admin_menu() {
add_menu_page('Vigtigt', 'Vigtige beskeder!', '1',
'hello-world', 'hello_world_html_page');
}
}
/*———————————————————————*/
/*———————————————————————*/
function hello_world_html_page() {
?>
<div>
<h2>Vigtige beskeder!</h2>

<form method="post" action="options.php">
<?php wp_nonce_field('update-options'); ?>

<table width="900">
<tr valign="top">
<th width="140" scope="row">Skriv din tekst her:</th>
<td width="700">
<input size="76" name="vigtigt_data" type="text" id="vigtigt_data"
value="<?php echo get_option('vigtigt_data'); ?>" />
(f.eks. Undervisning er aflyst pga. sne!)</td>
</tr>
<tr><td>

</td></tr>
</table>

<input type="hidden" name="action" value="update" />
<input type="hidden" name="page_options" value="vigtigt_data" />

<p>
<input type="submit" value="<?php _e('Gem ændringer') ?>" />
</p>

</form>
</div>

And the code from my home.php:

<div class='vigtig_besked'>

    <span class="knapoverskrift">VIGTIGT!</span>
    <span class="knaptekst"><?php echo get_option('vigtigt_data'); ?></span>

</div>