The most likely reason for this, is that your current focus is on the terminal. That is, you have an active cursor in the terminal, which will be blinking.
In order to save, simply change your focus by clicking somewhere in the text-editor pane, moving your active cursor to the file you wish to save. Now you should be able to save the file using any of the standard methods.
On Mac (and coming soon to Windows in v2.5) you can also use a keyboard shortcut to toggle your focus to and from the IDE terminal by pressing cmd+; (on Windows, the shortcut will be "ctrl+;").
If you are still unable to save after giving focus to the file, please see read this article.
IMPORTANT! Please do not attempt to "force" the file to save by closing the unsaved tab and triggering an alert that gives you the option of saving. This will write your edits directly to your local computer, and they WILL BE ERASED. The Learn IDE is a remote environment that places a copy of your remote work on your local disk. If you edit the copy, the Learn IDE will simply overwrite that copy with whatever version of the file is in your remote environment the next time it is able to (either when the file is altered in someway on the remote, or else the next time the IDE is launched). So while it may seem to have kept your changes for the moment, they are not in your remote environment, and they will be erased.