To restore an archived project, navigate to the project list on the left side of your dashboard (only desktop version). Hover over the word "Projects" until the small arrow appears, and click on it to open the project menu. Here you can switch from your list of active projects to your archived projects.
You will now see a complete list of projects that were archived, and from here you can delete or restore them. To do that click on the project info icon.
The next step would be to click on the ellipsis and select "Restore".
To restore a trashed task please click on Active in the top bar of the project, and select Trashed from the menu that appears (only desktop version).
On the next screen you will see a list of all your trashed tasks from that project and you can open and restore them. To do this, just click on the ellipsis and select Restore Task from the menu.