A method of assisting a pilot of a single-engined rotary wing aircraft (1) during a stage of flight in autorotation, said aircraft (1) including a hybrid power plant having an engine (13), an electric machine (12), and a main gearbox (11). Said aircraft (1) also includes electrical energy storage means (14) and a main rotor (2) mechanically connected to said hybrid power plant (5). According to said method, while in flight, the operation of said engine (13) is monitored in order to detect a failure thereof, in particular by monitoring for a drop of power on said main rotor (2), and then in the event of a failure of said engine (13) being detected, said electric machine (12) is controlled to deliver auxiliary power We to said main rotor (2), thereby enabling the pilot of said aircraft (1) to be assisted in maneuvering said aircraft (1) during said stage of flight in autorotation following said failure.