What happens to an unexecuted order after market closing?

For a regular day order, if it hasn’t executed by market close, it’s automatically cancelled by the exchange, and you have to place a fresh order the next trading session if you still want the trade. Some brokers let you place an order with a longer validity (like Good Till Cancelled or Good Till Date), which carries the order forward across sessions instead of expiring at close, though this depends on the broker and the specific order type supporting it. There’s also the After Market Order (AMO) route, which is a separate order type specifically designed to be placed after hours and queued for execution when the next session opens. You check the order’s validity setting when placing it, since a day order and a GTC order behave very differently once the market closes.