You have entered Vortex Ring State. Basically it is unsafe to descend a rotorcraft at a rate higher than 3 meters per second, and less than 50 km forward velocity; or you start descending into your own rotorwash and lose lift.
http://aerospaceplanes.suite101.com/article.cfm/vortex_ring_state
You can recover from VRS by nosing down/cyclic forward and increasing forward velocity to above 50 km/h..........assuming, of course, you have the altitude to recover!!