Perhaps the most confusing WordPress error that a beginner may come across is “Internal Server Error”, or sometimes “500 Internal Server Error”.
This error usually appears when there is something wrong, but the server is unable to identify where the problem is.
Solutions:
a) Checking for Corrupt .htaccess File
b) Increasing the PHP Memory Limit
c) Deactivate all Plugins
d) Ask Hosting Provider
http://www.wpbeginner.com/wp-tutorials/how-to-fix-the-internal-server-error-in-wordpress/