The Hindu scriptures propound the cyclic theory of creation. The universe has three cyclic states The cycle of sṛiṣhṭi (creation), sthiti (preservation) and laya (dissolution) goes on endlessly. The science also has 3 states of universe- Creation, expansion and destruction. Pralaya means the de-manifestation of the universe.

According to the Vishnu Purana, 1.7.38, there are 4 types of Pralayas:

1. Nitya Pralaya: The constant dissolution of all beings, like death that occurs daily. Everyone will witness it someday.

2. Namittika Pralaya/Maha Pralaya: It is also known as Brahma Pralaya and It occurs at the end of a Kalpa when Bhagawan Brahma goes into his sleep, which is equivalent to 4.32 billion human years. A great fire consumes all three lokas- the Bhu, Bhuvah, and Shah Loka. (Will write about lokas in details)

3. Prakrutika Pralaya: It occurs at the end of one Brahma ji lifetime equivalent to 10 [to the power] 36 human years. when the whole world (Brahmanda) reunites with prakriti. (Brahma is a designation, which keep changes with one entity to other with time).

4. Atyantika Pralaya: When man attains liberation he merges with Narayana. In simple words, attaining moksha is also a kind of Pralaya.

Final dissolution is called prakrutika pralaya. At that time all the evolutes are absorbed into prakrti.

Nature Of Maha-Pralay,  according to Mahabharat, shanti parva:

” At such a time the unmanifest and holy one urges the Being called Maharudra, who is conscious of his great powers, (for destroying the world). Maharudra, being assuming the form of Surya of hundreds of thousands of rays, divides himself into a dozen portions each resembling a blazing fire. He then consumes with his energy, without any loss of time, the kinds of created beings within the twinkling of the eye all mobile and immobile creatures being thus destroyed, the Earth becomes on every side as bare as a tortoise shell. Having burnt everything on the face of the Earth, Rudra, of immeasurable might, then quickly fills the bare Earth with Water possessed of great force. He then creates the Yuga-fire which dries up the Water (into which the bare Earth has been dissolved). The Water disappears, and the great element of Fire continues to blaze fiercely. Then comes the mighty Wind of immeasurable force, in his eight forms (astamurti tatva), who swallows up quickly that blazing fire of transcendent force, possessed by seven flames, and identifiable with the heat existing in every creature. Having swallowed up that fire, the Wind courses in every direction, upwards, downwards, and transversely. The space of immeasurable existence swallowed up that Wind of transcendent energy. Then Mind (awareness) cheerfully swallows up that immeasurable Space. Then that Lord of all creatures, viz., Consciousness, who is the Soul of everything, swallows up the Mind. Consciousness, in his turn, is swallowed up by the Mahat-soul who is conversant with the Past, the Present, and the Future. The incomparable Mahat-soul or Universe is then swallowed up by Sambhu, that Lord of all things, to whom the Yoga attributes of Anima, Laghima, Prapti, &c., naturally inhere, who is regarded as the Supreme and pure Effulgence that is Immutable. His hands and feet extend over every part; his eyes and head and face are everywhere, his ears reach every place, and he exists overwhelming all things. He is the heart of all creatures; His measure is of a digit of the thumb. That Infinite and supreme Soul, that Lord of all, thus swallows up the Universe, After this, what remains is the undecaying and the Immutable One who is without defect of any kind, who is the Creator of the Past, the Present, and the Future; and who is perfectly faultless.”

There could be 3 cases when discussing what happens during Maha pralaya.

1. Beings who have not attained moksha
During pralaya all such beings cease to have any experience and remain in a state of potentiality. An state which remain unexplained. The may or may not get another chance to attain moksha (DEPENDING ON THEIR KARMAS)

2. Beings who have attained moksha
People who have attained such a state do not depart anywhere and are not dependent on the state of the universe. They are unaffected by pralaya. Moksha is not dependent on time and is thus not affected by this pralaya.

3. Beings who follow the path of moksha but haven’t attained it.

The explanation of this case has been mentioned in Brahma Sutra Bhasya 4.3.10-

The one who haven’t attained the moksha yet after following the path, will be merged in Brahman i.e, will attain moksha.