The Tail Motor on the Blade MSR Helicopter is very fragile.
People say that if you hit something with the tail motor, the axel will push inside the motor and will pop off the black cap on the other side. In my experience, I think you don't even have to hit something. The cap will just pop by itself if you got a bad motor.
Fortunately, Blade support is great, and if you call them and explain the problem, they will usually send a new motor for free. In fact, the whore tail boom assemble come together, and it is very easy to replace.
The symptoms of a bad tail motor is either that the motor will not turn at all, or more subtil symptom is that the helicopter power shuts down completely after you push throttle. In the later case, the way to verify that the power shut down is caused by a bad tail motor is to disconnect the tail motor and see if the same behaviour is observed.
People on the internet recommend that the tail motor be reenforced before flying, with various tricks. Some just put tape around the end cap. Others will use shrinkable wire isolator, or a little ring around the motor to keep its cap in place.