How to Fix the Internal Server Error in WordPress
Check for the corrupted .htaccess file.
Login to your site using the FTP
Download the .htaccess file
Rename the .htaccess file and upload it to server.
Check if it is resolved or not.
If fixed the internal server error,then go to Settings » Permalinks and click the save button.This will generate a new .htaccess file
Increasing the Memory Limit
Insert the following code into Notepad.
memory_limit = 64M ;
Save as “PHP.ini”.
Upload this file to “wp-admin” directory.
Re-uploading Core Files
re-uploading the wp-admin and wp-includes folder from a fresh WordPress install
Deactivate all Plugins