Clutch not disengaging when engine is on

This morning, I shifted into neutral and left the truck running.  When I got in it put it in reverse, the shifter peddle went all the way to the floor and the clutch did not engage.  I turned off the engine, and the transmission shifted easily.  I put it into reverse, started the engine, and when I tried to shift back into first, the same thing happened.  Peddle to the floor with no resistance, and I could not get it into gear.  I finally forced it into 1st to get back into the driveway, but had to stall the engine to keep from going through the garage door.  The clutch would not disengage at any time.  I checked the master cylinder and it is 1/2 full.  Has my clutch gone out?