You opened your website in the browser and were surprised to see WordPress 500 Internal Server Error instead of your website.

To resolve the issue, let’s first check the error log file.
* Login to your host via Control Panel (cPanel / Plesk).
* If your host uses CPanel, then go to File Manager else if your host uses Plesk then go to Log Manager and open error_log file.
* Search for the error messages like: “Premature end of script headers“.

This issue can be resolved in a number of ways. I am providing here all the methods.
You are free to use any one depending on your choice.

Solution 1: Increase the Memory Limit of your blog using PHP.INI file
Memory limit that your website can use is set in php.ini file at the below line
memory_limit = 32M;
Now change it to 64M or even higher if you want.
Reload your website.

Solution 2: Increase the memory limit of your blog using .htaccess file
Open .htaccess file and add the following line and save the file.
php_value memory_limit 64M
Reload your website.

Solution 3: Delete .htaccess file
* Go the root directory of your blog and delete .htaccess file.
* Login to your WordPress Dashboard and go to Settings –> Permalinks and Click on Save button. This will again generate your .htaccess file.
* If you are using WP-Super Cache Plug-in, then go to Settings –> WP Super Cache. You would see some code that to be added to .htaccess file. Click on the button to add that code to your newly created .htaccess file.

NOTE: Make sure to back-up your .htaccess file before making any changes to it.
Prior to this, I shared one post regarding How to redirect Add-On Domain.