There are many reasons may be the reason for the 500 error. You can follow some tricks to resolve this issue. Before doing anything just backup your whole website to your local hard drive. After that just do what I am saying. Do follow these guides one by one.
1. Just Check Port Block to your Cpanel. Login to your Cpanel by specific user Id and password which you have already. Then search for Port Block Check. Here you will find Outbound Port 465 and 587 for your account are BLOCKED. If your website got infected with malware, probably you will face this 500 Error. You can see all the listed files which have got infected with malware. You have to scan all these files by antivirus OR if you don’t want that files, just delete. Before delete just check properly whether it will be needed for your website or not. My recommendation is to first scan by antivirus.
2. PHP memory limits: WordPress will attempt to extend the limit if you begin exceeding it, but it can only work as high as the limit your hosting provider has offered. If You need to increase the PHP memory limit in WordPress and refresh your site, then the issue may be resolved. Now the question is how to increase the memory limit. For this, again login your CPanel and open wp-config.php file. After that You have to put this code – define(‘WP_MEMORY_LIMIT’, ’64M’); You can increase the number from 64 to 128M or 256 M.
3. Another issue may be plugins which you have installed in your WordPress. For this, You have to log in to your WordPress and go to Plugins ( If you can access the WordPress ), then deactivate the plugin one by one. By this way, you can understand for which plugin the 500 error is showing. If you can’t access WordPress then do this process: Go to Cpanel and then Open File Manager. Open it and go to the public.html. Here you will find the specific website folder. Click on it and go to Wp-content and then just do rename the plugin folder. If the error is still the same then rename the plugin as it is.
You can do all these by using FTP also.
If your website is still showing error 500 then just call your hosting provider. They will definitely solve this out.