Sudden Hard Brake Pedal

You didn't mention the car make and model, but I'm assuming your car uses a vacuum powered brake booster. (See below image) It uses vacuum power from the engine to multiply the force applied to the brake pedal. If the brake booster fails or if there is a vacuum leak, it will not work properly, resulting in a hard brake pedal.

The brake booster (black), the brake master cylinder (silver) with two brake lines and a vacuum hose from the engine providing vacuum pressure.

If a component in the power braking system fails, you will still be able to brake, but only with your own physical strength. This is a big safety issue, so you should never drive a car without a functioning power braking system.

(You can try this yourself: After driving a car, when you shut down the engine, you feel the brake pedal is still soft, but after you press the brake a few times without the engine running, the pedal gets harder, because all vacuum pressuse gets released when pressing the brake.)

I'm not quite sure what you mean by "the brake fluid was completely dried up"? If it means there is no brake fluid in the brake fluid resevoir (above the master cylinder, see above picture), you can have air in the system. This does not cause the brake pedal to feel hard, it causes the brake pedal to feel extremely soft and will result in a loss of braking power or cause the brakes to fail completely. If you have air in the system, you (or a mechanic) should bleed the brakes.

But again, I'm pretty sure the issue is caused by a vacuum leak. The brake booster gets vacuum pressure from the engine intake. Whichever part is leaking must be replaced.