If your WordPress sidebar is appearing below your content instead of alongside it, there are a few common reasons and solutions you can try:

1. Incorrect HTML/CSS: This is the most common reason. Sometimes, incorrect HTML or CSS in your theme or a plugin can cause this issue.

   • Solution: Review your theme files, particularly, and make sure there are no CSS rules causing this. Also, check for any custom CSS you might have added.

2. Container Width: If your main content area is too wide, it can push the sidebar down.

   • Solution: Adjust the container widths in your CSS or theme settings to ensure there's enough space for both the content area and the sidebar.

3. Widgets or Plugins: A misconfigured widget or plugin can sometimes disrupt the layout.

   • Solution: Temporarily deactivate all your plugins and see if the issue persists. If it doesn't, reactivate them one by one until you find the one causing the issue.

4. Incorrect Template Files: If you've modified your theme's template files, especially single.php or page.php, you might have inadvertently caused this.

   • Solution: Compare your modified template files with the original ones from the theme and look for any discrepancies.

5. HTML Structure: Incorrectly nested HTML elements can sometimes cause layout issues.

   • Solution: Check your theme's template files and make sure the HTML structure is correct.

6. Custom CSS or JavaScript: Custom code added to your theme or through a plugin might be causing conflicts.

   • Solution: Review any custom CSS or JavaScript you've added and check if it's affecting the layout.

7. Theme Update Issues: If you recently updated your theme, there might be some compatibility issues.

   • Solution: Contact the theme developer or check the theme's support forum for any reported issues after updates.

8. Browser Cache: Sometimes, your browser might be caching an older version of your site.

   • Solution: Clear your browser cache or try accessing your site in a different browser or incognito/private window.

9. Check for Responsive Design: Ensure that your theme is designed to be responsive. Some themes might switch to a single-column layout on smaller screens, which could push the sidebar below the content.

   • Solution: Review your theme's documentation or settings to ensure it's designed to be responsive.

10. Child Theme Issues: If you're using a child theme, make sure any customizations you've made don't conflict with the layout.

• Solution: Review the code in your child theme and check if any changes are affecting the layout.

Remember to always back up your site before making significant changes to your theme or plugins. If none of these solutions work, you might need to seek help from a developer who can look into the specific code of your site.