The first thing you should do is check both your Trade Log and your Account window. This is to determine if the order was executed.  If executed, it would eventually leave the Order Management page. The Trade Log window should reflect executed orders, and the Account window should reflect resulting positions (or lack of position, if the executed order was to close a position). 

If the order was not executed, the next thing you should do is click on the tab on your Trader Workstation named "Pending (All)". After a period of time, open orders on the Order Management or Trading page will relocate to the Pending (All) page, which shows all pending orders. This could occur after a log off and back into the system, the nightly reset, etc. It could simply be that your open order has now left the Trading tab and is in the Pending (All) tab.

If the order was not executed, and also isn't appearing on the Pending page, the next logical consideration would be that the order was canceled. The order might have been canceled by the user or the IBKR system. Reasons for an IBKR system cancel are numerous, but a few of the more frequent examples are: order cancellation due to lack of funds in account to support all open orders; the order was submitted as a "Day" order instead of a GTC order and was appropriately canceled at the end of that product's day session; the order was part of a bracket or OCA group, and was canceled as a result of another order executing or the parent order being canceled. The aforementioned cancel reasons are not intended as an all-inclusive list, but rather a sampling of some of the more common reasons. You should contact IBKR as soon as possible, and one of our CSRs can research audit trails to try and determine the reason for the cancel.