You can see a list of tasks you completed on that day in your day view (at the very bottom - don't see it?).
So if you just marked a task done it will show up in the "completed tasks" section for that day (unless it was scheduled for another day, in which case you will find it in that day's completed task section).
Unmarking it done
You can then click on the checkbox icon again to unmark it done.
Alternative using the archive search
If you don't see the task there, you can also go to the archive (menu at the top right) and then click the magnifying glass icon there.
Since tasks are sorted by "most recently edited", you should find the task you just marked done at the top of the list! Otherwise you can search for it. Note: you'll have to be in archive mode in order to mark the task not done.