Can a commercial aircraft be detected by ATC's / Radar systems if all its engines fail?
In an event of failure of all engines of a commercial aircraft in mid air ...


That would be a very rare event. In the modern cases where this has happened within an ATC area, it has not prevented location of the aircraft so far as I know.




  ... and no secondary backup or generators powering the plane


Even a two-engined airliner has an Auxiliary Power Unit (APU) and a Ram-Air Turbine(RAT). I believe they also typically have 10-15 minutes battery power for critical systems. I don't know if any airliner has ever lost all generators other than in cases where the aircraft was destroyed in flight (e.g. PanAm 103).




  would the ATC still be able to detect the aircraft with accuracy 


If an aircraft's transponder and secondary-radar are not operating, ATC would rely on primary radar, which is less accurate, particularly for altitude. They would also ask the pilots for their position over VHF radio and ask other aircraft in the area to locate the aircraft.



Example: British Airways Flight 9

All four engines flamed out due to ingestion of volcanic ash. 


  Despite the crew "squawking" the emergency transponder setting of 7700, the aeroplane could not be located by Air Traffic Control on their radar screens.


The pilot communicated with ATC over VHF radio and later restarted the engines.


  The engines had enough electrical power to restart because one generator and the onboard batteries were still operating




Example Air Canada Flight 143 (the "Gimli Glider")

This 767 ran out of fuel mid-air and all engines stopped. 


  The 767 was one of the first airliners to include an Electronic Flight Instrument System (EFIS), which operated on the electricity generated by the aircraft's jet engines. With both engines stopped, the system went dead, leaving only a few basic battery-powered emergency flight instruments. 


The pilot communicated with ATC Winnipeg and


  First Officer Maurice Quintal began to calculate whether they could reach Winnipeg. He used the altitude from one of the mechanical backup instruments, while the distance traveled was supplied by the air traffic controllers in Winnipeg, measuring the distance the aircraft's echo moved on their radar screens.


So yes, If the aircraft with all engines out is in radar range and ATC are attentive, they can track the aircraft's location.