I placed a bid/sell order. Why didn't it happen?

Limit Order is a type of order where you can trade by typing the price and amount you want. Your order will wait on the board unless there is a seller at the buy order price you set or a buyer at the sell order price you set. For this reason, there is no guarantee of fulfillment of limit orders.
To cancel your open order, you can delete it with the CANCEL button under the My Open Buy Orders and My Open Sell Orders headings at the bottom of the Stock Exchange page.