I have been working on this project where the client wanted to be able to upload and install image galleries, so I have worked with this one and I love it: http://alexrabe.de/wordpress-plugins/nextgen-gallery/

All was smooth and working until, I removed this one line of code from my header.php in my WordPress theme directory.

<?php wp_head() ?>

Without this function in your header.php the gallery does not know where to insert the CSS/JavaScript files to create a fully functioned gallery. Be sure that it is placed before your closing tag.

To find out more information on the function check out this

All in all, be sure if you are running any type of plugin where they need to insert code upon plugin creation that you have that line within your head element of the page.

This problem has happened to me many times with my personal site as well as client sites. After trying different things and looking at other articles what worked for me was to:

• First log out using this url: http://www.yourdomain.com/wp-login.php?loggedout=true
• Rename your plugins folder to “_plugins” to back them up and create a new folder called “plugins” – this insures that there is on conflict with any plugins that are being run
• Replace your current wp-login.php file with a new one
• NOTE: You might get a header php error, and if you do follow this article posted at WordPress

Hopefully those steps can fix this problem for you. If something else works for you, feel free to post the results.

The basic concept, will go as follows:

• Create a new file and give it a name such as “allposts.php”
• Paste the code below in the “allposts.php”
• Upload the file to your theme directory
• Create a new page in WordPress
• To the right where you see Page Template, select the one you just created
• Save and view the page. You should now be viewing your posts

The code that goes in to the “allposts.php” is below, this is an example from my site. The XHTML can be customized to the theme of your website.
The top section of code is where the name of the page template is defined. This will display on the page editor under the Page Template drop down.

<?php
/*
Template Name: Page of Posts
*/
?>

<?php get_header(); ?>

			<?php
				$wp_query = new WP_Query();
				$wp_query->query('showposts=4'.'&paged='.$paged);
			?>
<section>
	<article>
			<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
				<h1><?php the_title(); ?></h1>
				<h5>written by: ryan coughlin</h5>

				<?php the_content(); ?>

				<?php edit_post_link('Edit this entry','','.'); ?>

				<details>
					<ul class="tags">
						<?php the_category(', ') ?>
					</ul>
				</details>
			<?php endwhile; endif; ?>

			<div class="page_navigation">
				<div id="p_next"><?php next_posts_link('« Read Older') ?></div>
				<div id="p_previous"><?php previous_posts_link('Read Newer »') ?></div>
			</div>

	</article>
</section>
<?php get_footer(); ?>

<!--?php query_posts("showposts=10"); ?-->

Lastly, once this file is saved and uploaded, go to your page you created and on the right side under “Page Templates” select “Page of Posts”

To read more on the WordPress query_posts(), check out the reading.