Another good example of terminology clouding the issues
"Tail Spin" is what stiff wingers do between Immelmann's and stall turns when showing off!
We fling wingers should call it one of the new fandangled name for it depending upon what you have experienced:
Loss of tail rotor control: You are not able to control the tail rotor pitch mechanism.
Loss of tail rotor thrust: Little spinning thing at the back stops spinning or falls off.
Loss of tail rotor effectiveness (LTE): "Newly" discovered and named in the 80's after many (in particular OH-58/B206) accidents. Although somewhat awkwardly named (as the tail rotor is still effectivly working and must be providing thrust) LTE refers to what is thought to be an injestion of main or tail rotor vorticey through the tail rotor which causes an onset of yaw in the direction induced by torque that cannot be overcome by the application of full "power pedal". The yaw rotation can build up quickly enough to fool most pilots into believing they have experienced a loss of tail rotor thrust. The concept has come under fire lately because of the early thoughts that the tail rotor enters vortex ring state being a little hard to proove. Oh - and then there is fenestron stall that possibly fits into this category too, although strongly denied as a possibility by the manufacturer whilst alledgedly being strongly experienced by the pilots!
Loss of Tail Rotor Authority(LTA): Also a new term to make the old Huey war story of "..and then I ran out of bloody left pedal and..." sound a little more sophisticated and technical. In this situation, the tail rotor does not produce enough thrust to counteract the torque/crosswind combination you require, your power pedal hits the stop, and around you go - though often quite gently when compared to LTE or loss of thrust. A lot of aircraft are susceptable to this, but the UH-1D/H Huey is famous for it - and many people have had the earth come up and smite them as a result. Someone mentioned the BK117 - it too is quite susceptible to LTA unless fitted with the C model tail rotor.
Thus BMK, you will need to give us a lot more info on your aircraft type/model, AUW, DA, manouevre you were attempting, wind strength and direction and IAS before we bull$hit artists can possibly hope to have a stab in the dark and argue around and around in circles about what caused it.