If i miss a important bug and i realise it very late then what will i do?

When your approach towards the testing task is really systematic there is no question of missing important bug araises. still i would prefer to answer your question.

If you really come to know that you have missed an important bug while reporting them Then you should react according to your cadre i.e. if you are a team member of a testing team then you should report the same to your Team Leader and he should take the necessary action for reporting that bug.

OR if your work environment permits you to report it to the concerned person then you should Log that bug and should check for the same in the relevant build