If your Base Turret stopped firing, it most likely means you selected Unit Shield, EMP, or any of the weapon cards. Please note that once you select one of these cards, you have to manually select a target in order for it to activate. For example, if you selected Unit Shield, you have to manually pick one of your own troops to activate the Unit Shield on. If you do not manually select a target, the card will not activate and your Base Turret will not fire.