You can find the scanned order by searching in the Orders section. It will have been moved to the Scanned list and it's status should reflect this.
Select the order by clicking any empty area in it's row, the row will change to green, and choose Awaiting from the Status drop down. This will change it's status to Awaiting and place it in the Awaiting list. (It's also possible to right click the Status tag directly and choose another status from the options shown). All items in the order will be set reset so they can be scanned again. The operation will be logged in the order's activity log showing who moved the order back to Awaiting.