Make sure you put enough fluid back in it. Usually just dumping the fluid out won't hurt anything immediately. The clutches work off hydraulic pressure, and if there is no fluid, there is no pressure, which means there is no clutch apply. If this just happened in the driveway, I seriously doubt it hurt it, unless the car just ran for an obscene amount of time with no fluid. If that was the case, it would have damaged the pump from no lube. Dump 3 or 4 quarts of fluid in it, and see if it doesn't start working.