There is nothing to worry about! Just go to
Launchpad -> Utilities-> disk utilities click on disk utilities.
Now you will see your hard disk partition that you have ejected (faded). right click on it and choose mount. You are done, your hard drive is back.
Launchpad -> Utilities-> disk utilities click on disk utilities.
Now you will see your hard disk partition that you have ejected (faded). right click on it and choose mount. You are done, your hard drive is back.