In what episode did they come back? The Cybermen have returned many times, try and narrow it down for us.
Do you mean in the new series? If so, they are actually a separate species/version developed in an alternate universe and invaders into ours. They do not have the same origin as those in the classic series.
- And the return of the original Cybermen in "A Good Man Goes to War" is just a return after a period of time; unlike the Daleks there has been no "final battle" scenario chronicled in the TV series or movies, so the gap between "Silver Nemesis" and AGMGTW is just a matter of time rather than any sort of circumstance from which they had to "return".