My worldly understanding of Nirvana is like this, AFAIK Buddha has three forms i.e. Nirmanakaya (body of transformation), Sambhogakaya (body of bliss) and Dharmakaya (body of essence also called truth body, unmanifested mode).
I believe after Sakyamuni Buddha attained Nirvana, he uses his Nirmanakaya body for us to perceive through our physical eye. Because everything conditioned is impermanent, Buddha's human body also has to cease but this change does not mean change to suffering since it's free of greed, hatred and ignorance.
Sakyamuni Buddha may manifest in his unmanifested form at the moment (in this kalpa) and by using the nature of impermanence he may manifest to different form as he wish.
This is just my understanding