What follows assumes that you have been using Time Machine, backing up to that external drive.
The easiest way would be to connect the external drive, then reboot into Recovery HD. When you restart and you hear the chime, hold down the option key until the screen appears with the different drives displayed. Click Recovery HD.
In the screen that comes up, choose Restore from Time Machine Backup. You will then be given a choice of full backups to restore from; you would try the most recent one first.
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