There are two major reasons that cause HTTP Error 500 in WordPress. The first one occurs when the .htaccess file is corrupted, and the second one is the memory limit of PHP exceeds. The internal server error code 500 is seen on every page of the website when you find some issues with the file system or system that powers the website. The major responsible cause that occurs is due to a fault in the root directory where WordPress files locate. Another reason might be the problem with the server of the host.
Ways to fix internal server 500 error
Take a backup of your website.
Use An FTP Client.
Create a new file with .htaccess extension.
Increase the limit of PHP memory.
Less often used solution:
Find out a plugin that has a fault
Locate issue after debugging the website
Check whether all folders and files have right permissions
Upload wp-content and wp-includes folders to the website.
To know in detail, get in touch with the WordPress professionals