Sometimes wordpress themes are poorly coded. Some plugins require very specific structure of wordpress themes to run properly. In such cases plugins might not need attention but your theme.
Your first try should be deleting all plugin files and reactivating the plugin, if this doesn’t work you can look forward to below things in your theme.
A theme’s </head> tag should end with wp_head() php function. wp_head() function adds additional wordpress header functions to the site. Some plugins which modify header of your blog uses the function to inject functionalities. e.g. All in one SEO. So wp_head() must be called before closing head tag.
<?php wp_head(); ?>
Theme’s header should be called by <?php get_header(); ?> . The same functionality can be achieved by include function of PHP, but get_header function indicates the header file header.php will be included there and so does other related functions.
The same is with sidebar and footer files. A theme should call sidebar by <?php get_sidebar(); ?> . It will call sidebar.php and all sidebar widgets. To fix sidebar widget issues you need to apply this fix first.
Footer file is called by <?php get_footer(); ?> which enables additional footer functions(if any) and inclusion of footer.php
A wordpress theme usually has following files, which is not mandatory but recommended.
Single.php – Single post template
Index.php – Index page
Footer.php – Footer template
Header.php – Header template
Page.php – Page template
Search.php – Search Results
and many more.