Imagine that a helicopter raises its nose up and then there is a loss in speed. Can it enter a stall? If so, how do you recover from the stall?
Yes and No.
The definition of a stall is that the airfoil stops generating lift, which happens when the critical angle of attack is reached.
In an airplane, this happens during normal flight by pitching up until the critical angle of the airfoil is reached.
In a helicopter, aft cyclic ("pitching up" in an airplane) will only serve to make the helicopter climb and slow down until it reaches equilibrium in the new attitude.
Helicopters, however, can suffer from what is called "retreating blade stall," when the blade is at full pitch on the retreating side of the rotor:
This doesn't happen under normal circumstances, but there are four factors which contribute to retreating blade stall:
fast -- close to Vne
heavy -- at max gross weight
hot -- above 35C
high -- at high altitude
Retreating blade stall is considered an emergency, and can be recovered from by reducing power.
A stall, as in slowing down and then descending too fast - yes, in the following scenario:
- Helicopter flies with forward speed, above the hover ceiling.
- Pilot moves cyclic to aft, and pulls up collective to maintain height.
- When all airspeed is gone, the helicopter cannot maintain hover at this weight/altitude combination and starts to lose altitude.
- Descending vertically, the helicopter descends in its own downwash and enters a dangerous situation called Vortex Ring State. Even if more collective travel was available, it would not create more lift in this state.
There is a discussion possible about what stall exactly is. If it is flow separation, the rotor blades are technically not stalled as @Jan Hudec points out. If it is stopping the generation of aerodynamic lift, this is exactly what happens during both fixed wing stall and rotor wing vortex ring state.
Even the way to get out of vortex ring state is the same as with fixed wing stall: move the cyclic forward so the nose tips down. Or sideways alternatively left/right and descend like a leaf from a tree.