Corruption in .htaccess file caused this error.This is a simple htaccess error.
To fix 500 internal server error in wordpress
1.Save a copy of (backup) your .htaccess file.
2.Open your .htaccess file.
3.Delete all lines from your .htaccess file and save the file.
4.Login to wp-admin. In your admin dashboard click Settings and select Permalinks.
5.Set your permalink structure and click save changes.
6.If you have any other information in your htaccess file other than wordpress (ie.) manually added by you,add it again from your old .htaccess file.