On the webpage if you check a box it goes away forever!
I mistakenly checked all my tasks thinking the boxes to the left were simply to select them, as I was going to try to do a mass update to the due date, and they all went away forever! In this day and age of forever information, I'm disappointed because this is the second time I've made that mistake and in effect this has REDUCED productivity for me because the dozen or so things I had to do today, there are probably a few that I will forget and be pissed about later. WTF!
Keep cool, it has not gone away forever.
If you click on a project (or on the inbox), you will get the list of active tasks in that project. At the bottom-right of that list, there is a small icon that looks like a watch with an arrow that turns counter-clockwise. The tooltip for that icon is "Show accomplished tasks".
Just click on that icon. You will see the tasks you were not able to see. You will be able to uncheck them.