why ongc share price fall down suddenly on today?

Answer 1

ONGC has declared issue of bonus shares in the ratio of 1 bonus share for every 2 shares held. Yesterday was the last day to avail the bonus shares. Today the market prices has been adjusted for the issuance of the bonus share. That's the reason why you are seeing a markedly downward price of the ONGC scrip.
If you are holding some ONGC shares , don't worry about this price. You will be compensated by additional shares that you will get as bonus.