If you do not buy, exchange will automatically buy it for you around 3:20-3:25 PM at currect market price. She is a bit** lol
usually broker buys it back for you at 3:20 pm, however, its best to confirm it by asking your broker directly. different brokers have different rules.
Many brokers including zerodha, allow you to short sell the stock, with the condition, that if you dont buy back the stock before end of day, it will be automatically bought from your trading account during close of trade that day.
If even the auto square off is not possible, like trading halt on that particular stock due to upper circuit or lower circuit hitting, or if no sellers are available, then T+2 day, u will be taken to auction market, where those many shares are to be forcibly purchased by you, at whatever auction price is. You will be levied additional penalty also.
If you do so, your short position will be then auctioned by the respective exchange and will be bought at whatever the price the script is on T+2 day with some penalty, usually heavy on your pocket. This is only applicable for shares (cash segment). However, you can hold overnight positions while trading on F&O (Futures & Options). And always remember to buy back the stocks you have shorted before 3:10pm. 