Menu is not styled (WordPress)
If the images in your slider are displaying below each other in a row, or nothing shows up at all, check out this documentation.
Missing wp_head() or wp_footer() callings
Go to your theme's header.php file, open it, and search for for wp_head(). If you can't find it, put it right before the closing head tag.
<?php wp_head(); ?> </head>
Then open your theme's footer.php file, and look for wp_footer. If you can't find it, put it right before the closing body tag.
<?php wp_footer(); ?> </body>
You might have a large HTML error in your website. If you have Firefox browser downloaded, open your page and right click -> View page source. Look through the code, and if you see something marked with red, you have a HTML error, which you should fix.
It is possible, that you won't see any red codes, but you might still have HTML errors.
Wrong page start
Make sure that your website starts with something like:
If your page does not start with that, open up your header.php file, and make sure that you find that in the top of the page.
It is possible that the theme developers inserted some comments to the page's header, so it is not a problem, if the page starts with something like this:
<?php /** * The Header template for our theme *
Basic HTML codes are missing
Make sure that your website's source has these codes:
<html> <head> </head> <body> </body> </html>
in this order.
If one of them is missing, check out the header.php (for the opening html, the head and the opening body tag) and the footer.php (for the closing body and html tag).
PHP error caused by the theme
If your theme's file are containing the codes above, yet your page source shows that the closing body and html tags are missing, open up your footer.php file, and search for the last HTML code you found in the page source. For example, if your site ends with this code:
your footer.php file probably will have some PHP codes after it which might fail causing the file to stop before its end.
<div class="copyright"> <?php some_theme_function(); ?> </div>
You should simply comment it out:
<div class="copyright"> <?php //some_theme_function(); ?> </div>
For a faster result, you could put these codes to your theme's wp-config.php file:
which will write out the PHP errors, so it will be easier to figure out the problem.
If you checked the codes and they're all there, try to turn of all of your plugins, and check the page again (do a refresh on it). If the menu is correct now, one of the plugins were causing the problem, so what you should do is turn them back in small groups, and refresh the page everytime a group was turned back on. If the menu is wrong again, the problem will be caused by one of the plugins you activated for the last time, so deactivate those plugins one by one.