Notification Area applet is a nifty tool that lets you keep an eye on application activity of your Gnome desktop. Quite often new users tend to accidental it in the applet adding and removing frenzy that we all succumb to one time or other.
Here is how you can get the applet back on to your desktop panel. Select the 'Add to Panel' option by right-clicking the empty space before the clock applet. Scroll down and type the name of applet you want to add to the panel as show in the screenshot below.