↪ Replying to @lissijean
@lissijean Is it the external display still black when you close the laptop lid?
@lissijean If the external display works when your laptop lid is closed it's most probably a mirroring issue; you'll want to delete a preferences file.
@lissijean This is a good walkthrough. Resetting SMC has worked for me before. idownloadblog.com/2016/03/19/fix…