Sometimes plugins can conflict with a theme or with each other.  Disable all your plugins and see if the problem persists. If everything is working once the plugins were disabled it means there's a conflict with a plugin or maybe even a set of plugins. Enable the plugins one by one to identify the one that is creating the conflict.

Right click on the /wp-content/plugins folder and rename it plugins.old. This will deactivate all of the plugins. In most cases, this will also lock the WordPress admin area as well.  You will still be able to perform these steps from within the File Manager.Reactivate the plugins folder by following the above 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.