Don't rely on one parameter for a judgement.
The engine spools N1, N2 and N3 will windmill in flight at speeds sufficient to display RPM on the gages.
If you want to know if an engine is sick enough to be considered on deaths door look at EGT if its going up through the roof then that means there is no useful work being done and you better shut the fuel off.
The RPM loss to zero is typically a speed pickup/transducer failure and often associated with major fan damage associated with felt vibration in your seat and EGT through the roof. Damage in the high spool may or may not be felt as vibration, but EGT through the roof not responding to thottle retardation signifies major damage in this rotor.
note: the above is not meant to substitute for your training for a specfic installation and is only meant to help you understand what's likley going on in a failing engine.