I have a late-2013 27" iMac that I have hooked to a secondary monitor (Nixeus Vue 24"). The display was working without issues a few weeks ago, but I recently performed a clean install of 10.11.4 and my second monitor stops working after the computer goes to sleep overnight. I do have a second computer that uses this monitor as it's primary display but this did not have an effect on the setup before I reformatted. Has anyone else experienced this issue and is there a way to fix this without performing a re-install? If you need more information from me just ask.