First go to Settings and, at the far top left, click on "Recently Deleted Tasks". You'll see any tasks that you've deleted in the past week, including any subtasks.
If the missing tasks are not in this list then you haven't deleted them recently.
Next, try a Quick Search and, if that doesn't find the task, do a Search and specify whatever information you remember about the task.
You should also be able to see the reason the task was deleted in that same section. Unfortunately, there is no way to undelete a task. Sorry for the inconvenience.