Check Postgame guide for answers to most questions players might have
https://docs.google.com/document/d/1n0MGk8DAMW4lsbUh26LCoBtm1N0_AHQH_2ahvjaTAnY/edit#
Arclight is only in the nightclub after restoration and before you start the Devon Corp raid quest. If you already started the quest, you'll have to wait until after you get Titania's badge for him to appear there again.