I experienced the "500 Internal Server Error" earlier today on one of my sites powered by WordPress. In fact all addon-domains were also affected.
Instead of checking through the server logs for what might have caused the error, I contacted my host for support. The rep requested for my cPanel login details and within few minutes, he was able to detect and fix the problem.
He told me that the .htaccess file was misconfigured, which surprised me because I've not tampered with the file for a very long time. I assumed it might have been caused by the recent automatic upgrade of my WordPress installation.
So, if you're experiencing similar error especially after updating your WordPress, you might consider checking the .htaccess file config or request your host fix it for you.