The 500 Internal Server error is not always related to WordPress. The reason can be also other issues on the server.
Create a New .htaccess File
Choose the Files section and click on the File Manager icon:
On the riht upper corner click the Settings button:
From the drop-down menu select the domain name you need to access.
Check the "Show Hidden Files (dotfiles)".
Search for the .htaccess file in the list of files:
Right-click on the .htaccess file and Rename it .htaccess.bak.:
From the navigation menu in WordPress:
Click on Settings >Permalinks:
Click the Save Changes button.
Reload the site to see if the 500 Internal Server error has been resolved.
If the error appears again, try to increase the PHP memory limit. Please read this article to learn how to resolve the PHP Memory Limit Error.
If after increasing the PHP memory limit the 500 Internal Server error haven't been resolved, it may be an issue caused by one or more of the plugins installed on WordPress. Often times, if a new theme is installed, a previously installed plugin will not be compatible and will result in the 500 Internal Server error.
Go to the Files section and click on the File Manager icon.
Select Web Root (public_html/www) and the Document Root for the domain using the WordPress install, then click Go.
Click to expand the public_html folder, and navigate to the /wp-content/plugins folder:
Right-click on the /wp-content/plugins folder and rename it plugins.old.:
This action will deactivate all of the plugins. In most cases, this will also lock the WordPress admin area. You will still be able to perform these steps using the File Manager.
Reactivate the plugins folder by following the instructions and renaming the folder plugins. This will allow you to reactivate each plugin individually to isolate the offending plugin and resolve the 500 Internal Server Error. This should also allow access to the WordPress Dashboard again.
From the WordPress Dashboard:
Reactivate each plugin (one at a time) and refresh the website to see if the issue has been resolved.