Yes, you can cancel an active order at any time. In order to do this, please find the active orders section of the Trade page and hit the "Cancel" button. Please note the trade pair you are in, as the table is only populated by orders in the current pair.
Tip: Notice, that your Balance section is divided into several segments:
As you can see, total balance is divided into "Balance" (for available financial assets) and "In Orders" column designed for housing funds, temporary involved into trading activities.
Keeping it simple, when you place a trading order, the designated amount of funds is moved to the "In Orders" section. When you cancel you order, consider that funds from "In Order" section are returned back to the "Balance".
Be assured that your funds never got lost. They are just transferred between different sections of your balance.