enabling-the-login-form-module
Joomla! 3.x 
To enable Login functionality on your website, the Login Form module must be published. To publish the Login Form module:
- Log in to the Administrator back-end.
- Click the Extensions → Modules menu item.
- Locate the module called "Login Form".
- If the Status is a green tick, it is already enabled. If the Status is a red cross, it is currently disabled. Click on the red cross to enable.
To learn more see Changing the Login Form module settings;