If your order has been sitting in market for a few days, this is likely because you submitted a limit order outside of market hours and the price of shares moved when the market opened.
Orders will expire automatically after 7 days and if the price you submitted your order at matches the market price it will fill during this period. If it looks like your order is unlikey to fill or you want your order to go through sooner, you can always cancel your order and resubmit when the market is open.
To check the status of your order go to Portfolio>orders in your account.