When working on hydraulic clutch systems where the clutch pedal goes the floor, often just a few minutes of bleeding will fix the problem.
But when that does not work, it's a different story. Lots of times if the pedal just goes to the floor without much warning its going to be the master cylinder or lines have burst. Check around the clutch master cylinder for leaks. Sometimes all it needs is a few seals and it's good to go.
Have someone depress the clutch while looking for a leak in the lines. If you still have fluid in the system when they apply pressure fluid will come out where the line is damaged.
Another possibility could be that the reservoir may be out of fluid (which means you have a leak that needs fixing). Some more uncommon things that can happen is having a broken clutch release bearing or fork or release cable.
Pricing for CLUTCH HOSE (CLUTCH KITS & PARTS)
Pricing for CLUTCH MASTER CYLINDER & RELATED (CLUTCH KITS & PARTS)