Asking what happens to a deceased person who had attained Nirvana is a loaded question. It’s like asking an innocent man if he’s stopped beating his wife. If Nirvana is the Ultimate Truth, there can be nothing after it:
The Buddha was often asked this question of the state of a Tathagata, here meaning a Buddha or Arahat, after death: could it be said that he ‘is’ (eternalism), that he ‘is not’ (annihilationism), that he ‘both is and is not’, or that he ‘neither is nor is not’?
These were part of a small set of ‘undetermined questions’ which the Buddha set aside without answering (S.IV.373–400). One reason for this was that he saw speculating over them as a time-wasting diversion from spiritual practice.
When one monk told him that he would leave the Sangha unless he was given answers to these questions, the Buddha gave a simile to show how foolish he was: if a man was shot by a poisoned arrow but refused to let a doctor cure him until he knew everything about who shot the arrow, and what the arrow was made of, such a man would soon die (M.I.426–31; BW.230–3; SB.168–72).
The Buddha then said that he had clearly explained dukkha (suffering, dissatisfaction) and the way beyond it, but that asking the undetermined questions was not connected with, nor conducive to, Nirvana. This accords with his saying that he taught only what was both true and spiritually useful (M.I.395).
Besides these practical considerations, the Buddha also clearly saw the undetermined questions as having a misconception built into them. Like the innocent man who was asked ‘have you stopped beating your wife?’, he could not rightly reply either ‘yes’ or ‘no’ to them.