Technically, theres no perfect reason that causes this HTTP error 500. The only thing that is clear with this sort of error is that it is caused by any operation that went wrong at the sites end. But, certain troubleshooting steps are there following which this error can be rectified.
If you can access the dashboard then try to deactivate all your plugins and look for the result. If your website starts loading then it will be cleared that the error was because of any plugins.
Another troubleshooting instruction that will resolve this problem is to turn the debugging on. If this doesnt fix the problem, it may give more details about the error.
If both these solutions dont work then you will need to reinstall the Wordpress to fix it up. For this, you must follow the manual WordPress Update instructions from WordPress Codex.
