Once, what happened is I answered a question which obviously I thought as an appropriate answer to that question. But after some time, the question was edited by the 'asker' itself. The edit changed the slight meaning of the question. Although it didn't affect the question much. But what about my answer?. I got many down-votes for my answer (which was appropriate before the editing of the question). This is something unfair and shouldn't happen. Now, what can one do if He/She is in my position?
What you can (and should) do is edit your answer to match the updated question.
Now, if the question is edited drastically, so much that it's asking something completely different and thus invalidates any answers that have been posted, that's a different story. That's not supposed to happen. So in that case you can roll back the edit to the question. If you're not sure whether the edit is drastic enough to qualify for rolling back, you can ask for a second opinion in chat or here on meta.