Where has ancient technology knowledge gone? How did those technology ideas vanish?
The ancient technologies you mentioned like Shudarshan Chakra, Brhmastra, Pushpak Viman (Pushpak flying chariot) were divine technologies as stated in Hinduism epics and mythology. They were having their respected purpose and owners to whom these technologies were to be returned after serving their goal.
Divine Weapon Technology
The underlying principle of these divine weapons and technologies is the "Tejas" or the power with which they function. And it is supplied by the respected deity upon whom the name and function is assigned. Examples include Varuna Astra, Agni Astra, Narayana Astra, Pashupata Astra, and so on.
Normally these astras belong to their respective deities; for example, Pashupata Astra is owned by god Shiva, and his name Pashupati is assigned to the astra. Each of these deities has an abode, and these divine weapons originally reside in them. And these must be returned to their owners in order to safeguard them and to prevent any misuse or before or after the death of the earthly owner.
These weapons contain enormous amounts of energy, so to possess them a person should have the the capability to employ and hold these weapons and to use them, not only physical, but also mental and spiritual (i.e. a person should behave according righteously, acting in accord with divine or moral law and must have sufficient merit). These weapons each had their own specific purposes, and were employed only for a limited amount of time to serve the goal. Below is the quote from Vyasa the author of Mahabharata about the divine weapons .
कृतकृत्यानि चास्त्राणि गतान्यदय यथागतम् |
पुनरेष्यन्ति ते हस्ते
यदा कालो भविष्यति ||35 ||
Thy weapons, having achieved success, have gone away to the place they
came from. They will, again, come into thy hands when the Time for
their coming approaches. At the end of Mahabhrata war ,when Pandavas
were retiring to the himalayas for their final journey. Agni asked
them to leave the divine weapons along the way.
At the end of Mahabharata war ,when Pandavas were retiring to the Himalayas for their final journey. Agni asked them to leave the divine weapons along the way.
O Arjuna, and ye twins of great courage, listen to what I say! Ye
foremost ones of Kuru’s race, I am the god of fire. The forest of
Khandava was burnt by me, through the puissance of Arjuna and of
Narayana himself. Let your brother Phalguna proceed to the woods after
casting off Gandiva, that high weapon. He has no longer any need of
it. That precious discus, which was with the high-souled Krishna, has
disappeared (from the world). When the time again comes, it will come
back into his hands. This foremost of bows, Gandiva, was procured by
me from Varuna for the use of Partha. Let it be made over to Varuna
From the above, it's clear that the divine chariots were also originally owned by respected deities and belong to them, and must be handed back to their original owners afterwards.
Below is a quote from a PDF article DIVINE WEAPONS AND TEJAS IN THE TWO INDIAN EPICS
written by JARROD L. WHITAKER. PAGE 105.
One of the prime reasons of disappearance or withdrawal of these technologies from the earth is the age of kali or kaliyuga , which is believed to be started when Krishna (avatara of Vishnu) left the earth.
Thus, power (bala), intuition (buddhi), tejas, and foresight
(pratipatti) all decline due to a change in time (kala , yuga), and
this consequently causes a loss in the potential life expectancy (ayus
) of humans. The Sanskrit word sakti ´ or “energy” is employed in
the second passage as a general word for all energy-substances.102 The
epic authors are suggesting that in the world in which they live, Kali
yuga, no one can own or employ divine weapons of the calibre portrayed
in Dvapara yuga . This is because of a decline in the potential
amount of tejas any given individual can acquire.
The original purpose of these technologies was to re-establish the degrading dharma on the earth. These technologies were brought on earth by divine will in order to assist the process. So basically these technologies were mainly active and used for a limited time, and when their purpose was achieved, they were sent back to their original places, where these technologies were kept to be brought again in times to come for the good of mankind in future.
So the ancient technologies you mentioned, like flying aeroplanes and various weapons of destruction, were operational using divine energy or the Teja of the deity, and were returned to their original owner after serving their purpose here on earth in the state of the Tejas (divine energy) and their physical forms.
But not all the technologies were divine: Some technologies like catapults, cannons, guns etc. mentioned in the ancient epics of Hinduism are still present in its modern form. Also, many ancient texts were lost in the course of time. In modern times there is a lack of Sanskrit scholars who can translate these works, so many works have still not been translated to reveal the ancient technologies and their general use.