Can a helicopter 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.