Last night was first occurrence. My usual sequence is to open the case of laptop and press power on monitor. The system didn't wake. I had to power on the laptop (MacBookPro Retina 13" late 2013 or early 2014 (Mojave). The laptop screen remained black but the external Dell monitor performed as usual. The laptop connects to the Dell monitor with a thunderbolt/miniDP-to-miniDP cable. Forward to today: again, system doesn't wake when lid opened. I press power button, laptop screen stays black/blank and Dell screen is fine. This time I was positive I had left the system in sleep mode. I pressed MBPro power button; no image on either Dell or internal display. I pressed brightness function key on keyboard -- no change. In system preferences, I looked at power management...did not see causative setting. I looked at system log in console. I noticed several user agent and launchd entries in the log but didn't think related to power or display issues. With Sublime Text I removed several deprecated packages. Then I installed an update on Alfred that required and executed a sys prefs reload. I am certain I left the MBPro in wake, laptop lid open, when I went to fix lunch. I came back to black screens on both monitors. I tried hard wake (power button) several times. Nothing. Came to this forum. There is a nearby LED light with magnet strip, so I'll test. Then do more research.