To move an event from Archived to Active, go to your "Events tab", and find the event you would like to move under "Archived Events".
Once in the event Overview, in the upper right corner, beneath your profile name and picture, you will see "Registration Settings".
The third field from the top will be "Archive Date". You can either move the date back, or just delete the field completely. After you submit your event you will be active again!
Please remember that Activating your event does not change the day registration ends.