If you have migrated your Codeigniter project from Windows to Linux and it is giving error 404 on Linux system but same project was working fine on Windows, there may be case sensitivity issue in your filenames because Linux treats file code.php and Code.php of same folder differently .
Solution :
Change first letter of filename in uppercase of all files of models and controllers both.
Folder name should be started with small letter. But filenames of the folders should be started with capital letter.
models :
./Admin.php
./Employee.php
./Emails.php
./Contractor.php
./Index.html
./User.php
./users
./users/User.php
./users/Email.php
./employees
./employees/User.php
./employees/Email.php
controllers:
./Admincontroller.php
./Employeecontroller.php
./Mailcontroller.php
./Index.html
./Users.php
./users
./users/Usercontroller.php
./users/Emailcontroller.php
./employer
./employer/Employercontroller.php
./employer/Employer.php
