Crashing Helicopters

[goofball]

I've never seen a helicopter crash in person or even on TV for real, but I've seen them in movies/TV shows, and here's what always seems to happen.

The rear propellor is blown off (the one that counters the spin from the top one) and the helicopter slowly spins as it crashes to the ground.

Here's my question:

I know why it spins, that's simple physics. But, why does it have to crash to the ground? The top propellor is still working, that's the one that provides the lift. I see no reason that the helicopter can't hold or even gain altitude.

Where have I gone wrong?