If the clutch feels normal, ie; springs back all the way to the top, then I suspect that the friction disc has stuck (rusted) to the clutch or flywheel because water got onto it when you washed the car.

Put the car in 1st gear, start it up with the clutch pushed down and let it hop away down the road until it unsticks, if not, drive it home in 1st gear and bring it in to me tomorrow and I'll fix it.