Well…there are TWO problems. The fact that the tank is empty means that either the float valve is stuck in the “off” position or the the water supply to the tank is closed off. The fact that the toilet flush lever swings without resistance means that the lever linkage to the ball or flap valve is disconnected. It’s odd but not impossible that you should have two mechanical failures simultaneously. In my experience the situation you describe is isually due to a float valve stuck in the “up”, i.e. “off” position.