If it's a recip engine, unfeathering the prop usually creates enough torque to restart the engine, provided it is not badly damaged. Think of a rolling start with a clutch and manual transmission in a car or truck.
Single shaft turbines can also be restarted this way, because th prop torque is directly coupled to the compressor rotor. With a two- or three-shaft machine, no such luck, although the engine starter may be used.