If the engine of a helicopter fails, is it possible to land safely?
You're asking two different questions, actually.


If the engines fails in a helicopter, the rotor will usually rotate and the pilot can land the helicopter by autorotation. The record is from an altitude of more than 12km by Jean Boulet when he descended via autorotation as the engine flamed out and couldn't be restarted.
If the rotor seizes for some reason in a helicopter, there is no lift produced and it will fall down. For tilt rotors like V-22, if the conditions permit (if it is in aircraft mode), the aircraft can be glided like normal fixed wing aircraft.