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
- Open Notepad.
- 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