A method for assisting a single-engined rotary-wing aircraft in the event of engine failure, said aircraft including: - a hybrid power plant provided with a single main engine, with at least one electric machine, with a main gearbox and with at least one electrical energy storage device, - at least one main rotor that is driven in rotation by said hybrid power plant, said assistance method including the following steps: - a step for acquiring at least one monitoring parameter S of said aircraft in order to detect any failure of said main engine, - at least one step for monitoring at least one electric machine when at least one activation relationship comprising instantaneous values of at least one monitoring parameter S and its time derivative is less than a dedicated activation threshold Threshold i, where i is a positive integer varying from 1 to n, where n is a positive integer greater than or equal to 1, where Threshold i is an activation threshold, and where k i is a positive coefficient, - a step for assisting with the flight of said aircraft, said at least one electric machine delivering auxiliary mechanical power We to said main rotor in a regulated manner, thereby making it possible to assist the pilot of said aircraft during the flight phase following said failure in manoeuvring said aircraft, and - a step for deactivating said assistance step, characterised in that said method includes a dormant state, a standby state, a primed state and an activated state, said method being in said dormant state as soon as said aircraft is started, it not being possible for said monitoring step to be performed during said dormant state or during said standby state, it not being possible for said assistance step to be activated during said primed state, said assistance step being activated during said activated state, said dormant state including a pre-activation step and a first inhibition step for inhibiting said pre-activation step, said standby state including a step for priming said at least one electric machine, a step for synchronising said at least one electric machine, as well as a first exit step for exiting said standby state and a fourth inhibition step for inhibiting said priming state and for inhibiting said synchronisation step, said primed state including said monitoring step, as well as a third exit step for exiting said primed state and a second inhibition step for inhibiting said monitoring step, said activated state including said assistance step, said deactivation step, as well as a second exit step for exiting said activated state and a third inhibition step for inhibiting said deactivation step, said dormant state, said standby state, said primed state and said activated state including said acquisition step. - a hybrid power plant provided with a single main engine, with at least one electric machine, with a main gearbox and with at least one electrical energy storage device, - at least one main rotor that is driven in rotation by said hybrid power plant, said assistance method including the following steps: - a step for acquiring at least one monitoring parameter S of said aircraft in order to detect any failure of said main engine, - at least one step for monitoring at least one electric machine when at least one activation relationship comprising instantaneous values of at least one monitoring parameter S and its time derivative is less than a dedicated activation threshold Threshold i, where i is a positive integer varying from 1 to n, where n is a positive integer greater than or equal to 1, where Threshold i is an activation threshold, and where k i is a positive coefficient, - a step for assisting with the flight of said aircraft, said at least one electric machine delivering auxiliary mechanical power We to said main rotor in a regulated manner, thereby making it possible to assist the pilot of said aircraft during the flight phase following said failure in manoeuvring said aircraft, and - a step for deactivating said assistance step,