Why has your car run out of coolant?

Chances are, you have a leak somewhere. You can check your hoses and cooling system components for leaks, and repair them as needed. You can either do a visual inspection, or do the dye and blacklight test.

You may also not be changing your coolant out often enough. In the cooling cycle, coolant will change from liquid to vapor and back, with the inevitable result being that some of it will evaporate over time. Coolant doesn't last forever - it does burn out over time, and will need replacement.