It could be something as simple as two rival species putting their differences behind them and wars etc coming to an end as a result of that.
It could be something more complicated such as a complete change of regime going from dictatorship to democracy. (Now there's a miracle I'd like to see more often in certain places here!).
Whatever your miracle, there will be those who oppose it, denegrate it and those who welcome it, who've been working for it for years and so on. What you can guarantee with miracles is things cannot be the same again after they have taken place and your story should reflect that. Miracles =- big changes and big reactions to those changes.