It turns out you picked a particularly susceptible helicopter. Unlike other helicopters which are still largely mechanical, the EC135 has digital engine control. The simplest solution is to pick a different helicopter.
I suppose the main engine is still running, no reason for sudden falling, right?
I have some bad news. The EC135 uses a sophisticated computer to control the engine. Safety is typically guaranteed with multiple fully redundant computers and wiring paths, but if all electrical devices fail at the same time the turbine is no longer under control. To quote Wikipedia:
- If a total FADEC (Full Authority Digital Engine Control) failure occurs, the engine fails.
- Upon total FADEC failure, pilots have no manual controls for engine restart, throttle, or other functions.
No electricity, no engine.
Fortunately we can safely assume the turbine is at least engineered to fail safe, it won't spin itself out of control. Unfortunately that means "shut down". So yes, you lose main engine power.
Fortunately there will be some residual power in the turbine as it spins down. That plus the energy in the rotors can be used to autorotate as covered by GdD.
But... is the pilot still able to fly it? Is it probable he would land successfully? Does the helicopter need any electricity to stay flyable?
Yes, fly-by-wire helicopters are very rare. While they will no longer be augmented, stabilized, or power assisted, the mechanical linkages will still work. And they'll be able to autorotate, as @GdD described, and as demonstrated in this Smarter Every Day video.
However, sudden loss of engine power, and of all their flight control augmentation, and any electronic instruments, might confuse the pilot. An experienced pilot will still autorotate and land, but it probably won't be as pretty as in the video. I found an actual crash report of an actual EC-135. The experienced pilot thought they'd lost all engine power, went into autorotation, and landed with no injuries, but trashed the aircraft. In fact the engines were fine, but they lost their Stability Augmentation System. Reading that report might help with your book.
Helicopter pilots train for a sudden loss of power and emergency landing. If they still have control over the aircraft they will be able to land it.