뭐 했어요: means: "What did you do?" It's a past tense form. 뭐 할 거예요: means: "What are you going to do?" It's a future form.

V + -(으)ㄴ 거예요 used for the action what the speaker has some information. V + -었어요 used for the action what the speaker has no information. 뭐 한 거예요? : What is that you did? (The speaker saw some actions or heard some noises.) 뭐 했어요? : What did you do? (The speaker has no information)