Ensure that the Date Filter is set to a date period that contains your Task. The Date Filter can be found in the top right corner above your Tasks.
Or select Overdue from the Task Completion Chart
Step 1: Select the Task Tab from the interface
Step 2: Select the Overdue checkbox on the Task Completion Chart (Deselect the check box next to Open and Complete in the legend, to hide that task category from the chart and task list)