Imported products do not show up in frontend

First check this option in Product option

• are enabled
• have stock quantity > 0
• have stock availability = In Stock
• have visibility = "Catalog, search"
• have the correct website assigned to them
• have the right tax class associated with them

Solution 1 : Manage Products > Select All, then select the action Update Attributes and add the products to the correct website(s).
Left side Display this information :  
Products Information
Attributes
Inventory
Websites <-- select this option

And select Main website under the Add Product To Websites 
 

Don't forget to rebuild the indexes.


Solution 2 :

<?php
require_once('app/Mage.php');
umask(0);
Mage::app('admin');

$website_ids = array(1, 2); // I'm assuming your website IDs are 1 and 2.

$product_collection = Mage::getModel('catalog/product')->getCollection();
foreach($product_collection as $product) {
    $product->setWebsiteIds($website_ids);
    $product->save();
}

?>