Central heating and air unit has stopped working and is leaking large amounts of water what could this be?

Define "stopped working".

Where is the water coming from?

Also necessary to know if you have ever cleaned the drip pans drain line? Some times this line clogs up and the condensation builds up.

I'm guessing that the unit is in your attic? If it is, turn the breaker to the unit off, climb up and see if the drip pan is full, if it is, use an electrical snake to slide into the drain tube and break up any clogs.

Wait until the drip pan drains off and clean the pan.