Press the reload button/key for your platform
PlayStation: press Square
Xbox: press X
PC: press the reload key shown in your controls, usually R
Make sure you have spare ammo
If the weapon is empty, reload when prompted
If you are using a controller, check the button layout in settings if Square or X does not work
If reload is disabled, switch to another weapon and back again
If you are in a cutscene or certain actions, wait until control returns