The logical answer is, as the others have written, your pads are worn down and need replacement. However, this is normally a slow process and doesn't result in an immediate loss of braking. Consequently, I'd lean toward something else that caused this.
In my experience, almost all sudden brake failures are caused by something in one of the hydraulic circuits, and not in the actual brake rotor/pad assemblies. As previously stated, issues with the rotors or pads typically develop over time, but most importantly, even with one or two of your brakes inoperable, the design is such that the others will still offer at least some braking effort.
I would troubleshoot this as follows:
1. Check the level in your brake fluid reservoir. If the level becomes low enough, air can be introduced into the hydraulic lines. This can cause your pedal to go to the floor. Also, check where the vehicle has been parked to determine if there are fluid leaks. Often (but not always), a brake indicator lamp will illuminate on your dash when it senses a low level (or something else wrong with the system). If the level is low and you've sucked air into the system, you can't just add more--you'll have to bleed it out before the brakes will work again.
2. If the fluid is good, inspect the front brakes by removing the wheels and looking at the condition of the pads and rotors. Pads that are completely worn down can eventually dig into the rotors, cutting deep grooves and eventually destroying them. If this has already occurred, you'll need to replace the rotors in addition to the pads. Please note that on rare occasions, I've seen pads break and crumble apart without actually being worn down, allowing the metal backing of the pad to come into contact with the rotor. If this has happened, it will certainly grind like you described, and you'll likely have to replace the rotor as well.
3. Make sure the brake calipers are attached and aligned properly. This may sound silly, but I recently had the brakes start grinding on my Corvette, and before I could get the wheel off, I noticed metal shavings around the rim. When I finally removed the wheel, I noticed that one of the bolts holding the caliper in place had backed out and the caliper was jammed at an odd angle between the rotor and the inside of the rim. All of these components had to be replaced. In short, make sure all of the bolts are intact and all of the pieces are where they belong.
4. If none of these fix the problem, you'll want to take it to a mechanic. There are a number of other (less common) things that can occur--master cylinders can fail, wheel brake cylinders can seize in place, ABS systems can do weird things when in failure, and at one point I've even had a control valve within the master cylinder intermittently stick for brief periods of time. None of these is a particularly difficult repair, but if you're unfamiliar with what to look for, you could be chasing around in circles.
In summary though, if your foot goes to the floor, the hydraulics aren't working--either the fluid has leaked out, it has air in it, or one of the mechanical components through which the fluid flows has failed. I strongly suspect that if you ran your brakes all the way down to the backing, you would have known it long before now, and there would still be at least some level of braking remaining.
Best guess is that your pads and rotors worn down so far that the caliper piston popped out of the bore and that's why you lost fluid.
Uh, think????? Your pads are down to nothing......
