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 with the default expiry will be automatically cancelled 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 unlikely to fill or you want your order to go through sooner, you can always cancel your order and resubmit when the market is open.
You can change the expiry period when you submit your order, choosing between expiry periods of 30 days, 15 days, 7 days and end of day.
To check the status of your order go to Portfolio>orders in your account.