There's a number of reasons that a brake pedal could go all the way to the floor.
Low brake fluid caused by a leak in the braking system such as a brake line or brake reservoir or your calipers or wheel cylinders could be leaking.
Either way something is wrong and you should get your vehicle to a mechanic right away and cease driving the vehicle until the brakes are fixed.
If your vehicle is not leaking brake fluid or you're not losing any brake fluid then the culprit is almost always a bad brake booster which can cause your brake pedal to go all the way to the floor.
Air in the brake lines can also cause your brake pedal to go to the floor.
If you're unsure of working on brakes have a mechanic check it out.