I believe you have changed something either in your functions.php of theme or installed a new plugin so because of that you are seeing this error.
A 500 error means there is something wrong with a piece of code being executed, this would in your case (most likely) be caused by a plugin or your theme, the only way to test this is through deactivation.
I suggest to access your server via FTP, or a file manager in your hosting account’s control panel, navigate to /wp-content/themes/WhatYourThemeName/, open functions.php and remove the code you added recently. or find it the recent plugin have installed recently and deactivate that to check.
I personally would recommend installing WordPress locally and testing any changes there before you do it on your live site.