Error 500 is an error generated by the system any time there is an internal problem, because of which the Apache server cannot manage the page output. This problem is provided, most of the times, by a code error on php or htaccess.
The first thing to do is checking the error_log. This is a text file generated to survey any eventual code error and it stands in the directory where the problem lies. You can check it by using File Manager or FTP with a text editor.
If you cannot see the website's homepage, the error_log to check is placed in the cPanel systems /public_html/error_log: you will find there all the eventual problems that require to be solved to correct the error.
Sometimes, it may occur that a plugin or a module generates the problem and that the error_log is generated in a dedicated subdirectory where it is placed. To check all your website's error_log you have to access to your cPanel File manager and look for all the error_log files in your account, as you can see in the image below:
If you can't solve the problem in full autonomy, please open a ticket, reporting all details and describing all the checks you've already processed.