Actually, the conspiracy didn't fail. Their aim all along was to assassinate Julius Caesar, and they were successful in doing so. We don't know what they had planned for after the assassination. Who did they think would take control of Rome? How did they think the people of Rome would react?
In the play, we hear lots of dialogue about why they should kill Caesar and how they should do it, but we hear very little about what they're going to do afterward. In Act II, scene 1, Metellus does offer a suggestion:
O, let’s have him[Cicero]! Because his silver hairs
Will also give us a good opinion with the people
And buy men's votes to commend our deeds.
It shall be said that his judgment ruled our hands;
Our youths and wildness won’t appear at all,
But all be buried in his seriousness.
He wants to add Cicero to their number so that people might say it was all Cicero's idea and so that he can pay people to side with them. But the others veto this idea.
In the play, the conspirators don't seem to think about what to do once their plan has succeeded.
The conspiracy begins to fail in Act III with Mark Antony's famous speech, "Friends, Romans, Countrymen." It is in this Act, after Caesar's assassination, that the crowd is turned into a hostile mob against the conspirators and Brutus. In the next Act, the Second Triumverate(Octavius,Mark Antony, Lepidus) decides which of the conspirators shall live or die.Next, you see the conspirators doubting each other when Brutus and Cassius argue;Brutus accuses Cassius of greed.The conspirators then go to war on the plains of Philippi against the Triumverate. The battle is going in favor of the conspirators until mistaken information leads to Cassius committing suicide.Brutus continues fighting bravely even as the last conspirators begin to lose the battle. Then Brutus kills himself and the conspiracy fails. The real reason behind the failed conspiracy lies in their motivations-is assassination a worthy means to an end? The conspiracy is doomed to fail from the start. Good luck on your exam!
the concspiracy falls because the took popular desision
The conspirators fail in Julius Caesar because they do not manage to take decisive control of popular opinion after their murder of Caesar. As Casca, one of the conspirators, notes, the crowd is extremely fickle and easily moved if its emotions are affected. For instance, regarding Caesar's fainting fit, Casca says: When [Caesar] came to himself again, he said, if he had done or said any thing amiss, he desired their worships to think it was his infirmity. Three or four wenches, where I stood cried, “Alas, goodsoul!” and forgave him with all their hearts. But there's no heed to be taken of them; if Caesar had stabbed their mothers, they would have done no less. (Act I, Scene 2) The conspirators know they must have popular approval -- this is why Cassius is so keen to get the support of Brutus. Again, Casca sums the matter up: O, he [Brutus] sits high in all the people's hearts, And that which would appear offense in us, His countenance, like richest alchemy, Will change to virtue and to worthiness. (Act I, Scene 3) It is not in failing to recognize the need for popular approval, but in not knowing how to successfully get it, that the conspirators fail. The most fatal manifestation of this failure is the conspirators' decision to not only leave Mark Antony alive, but to allow him to speak at Caesar's funeral. Cassius wishes to kill Antony: I think it is not meet Mark Antony, so well beloved of Caesar, Should outlive Caesar. We shall find of him A shrewd contriver; and you know his means, If he improve them, may well stretch so far As to annoy us all, which to prevent, Let Antony and Caesar fall together. (Act II, Scene 1) However, Brutus will not hear of it, thinking that this step is ignoble: Our course will seem too bloody, Caius Cassius, To cut the head off and then hack the limbs Like wrath in death and envy afterwards; For Antony is but a limb of Caesar. (Act II, Scene 1) In the abstract, that might be a correct judgment, but the damage that the conspiracy might have taken from the ignoble assassination of Antony is probably much less than the damage the living Antony is able to do by whipping up the mob at Caesar's funeral.