Master cylinder is empty?
Air in the brake lines?
Top off the brake fluid, and bleed the brakes.
ADDED:
After replacing the brake pads, there is going to be some play in the brake pads to the rotors at first.
Press the pedal to the floor three times or so slowly to get the pads to seat.
If after five or so times, if the brake pedal still drops, you may need to replace the master cylinder...?