That means there's a leak in one of the two hydraulic systems. First, check the brake fluid reservoir to see if it is out of fluid. If it is, look under the car and behind all of the wheels for signs of wetness.
If the fluid is not empty and no external leak is found, suspect internal leakage inside the master cylinder. If there is an external leak, that must be repaired first, then the system must have the air bled out.
After an external leak is repaired, you should also be prepared to replace the master cylinder. A lot of crud and corrosion builds up in the bottom halves of the two bores where the lip seals don't normally travel. When something happens that allows you to press the brake pedal more than about half way to the floor, those lip seals travel over that crud and can be ripped. From then on the pedal will always sink to the floor. The same is true when bleeding the hydraulic system. Some people do that with a helper who presses the brake pedal down. They must be instructed to never push it more than half way to the floor. The exception is if it's a new master cylinder because there won't be any corrosion in it yet.