If an app or program, such as Sierra, will not show on the desktop when opened:
Left click twice in the lower right-hand corner of the taskbar. (To the right of the Date and time)
This will hide and then re-show all open items that are on the desktop. If you right-click once in the same place, it will bring up a small menu, allowing you to either peek at the desktop, or to outright show it. Left clicking, or selecting the option again after right-clicking a second time, will return all items to visibility.
If this process does not get the app or program to show, close the app/program and reopen it.