You can find tasks using the search function. In the web app and desktop apps, the search bar is located in the navigation panel, underneath your smart lists. In the mobile app, the search bar can be found by tapping the search icon in the top right hand corner of your home menu.
For now, search is limited to task titles, including tasks marked done. You can also search your archived tasks by tapping or clicking the "Archived" tab at the top of the search menu.