Go up to above the command window output, to slightly to the right of the middle of the tool ribbon. Click on Layout . In the menu that brings up, click in the Show section beside any items that you want to show up.
If your command window itself is not visible, then use your operating system facilities to check to see if you have minimized it. Those are going to depend upon your operating system.
In OS-X / MacOS, you would right-click on the MATLAB dock item, and at the top of the items that show up, there is a list of windows; click on one of them to unminimize the corresponding window.
In MS Windows, it depends upon which Windows version you are using. With more recent Windows, you can move your currsor to the bottom to bring up a command bar (if it is not already showing). Hover over the MATLAB icon to have Windows show small summaries of the active windows associated with MATLAB; left click on the one that looks vaguely like the command window.
