You should tell us more about your Linux and environment, and how you lost the applet in the first place, because without more info it will be difficult to help you.
If your desktop environment is GNOME then you can add applets by right-clicking the panel you want and choose "Add to panel...". I'm not sure which applet you need precisely. Sometimes new users accidentally delete the Notification Area from their panels, and if that's where your applet was then you can add it back by adding the notification area applet back to the panel. Also try running the command 'gnome-control-center' and see if that gives you something you can use.
I hope it helps.