I have a "mid-2012" MacBook Air 13" running Mojave 10.14.4
Since around Sierra or High Sierra time I have had a problem where frequently my external display does not come on when I plug it in or wake my laptop. It is inconsistent, but a significant proportion of the time, the screen does not wake.
Screen is a Dell U2417 connected via the TB/miniDP port on the laptop to either DisplayPort input (screen at work) or HDMI input (same screen at home).
- the MacBook recognises that the screen is attached
- screen shows in display preferences
- the desktop reconfigures as if screen connected (i.e. the laptop screen changes resolution to match the external screen, even though external screen does not wake up)
- if I switch the screen off and on it just says "no signal" and goes to sleep
- unplugging and re-plugging the screen does not fix the problem, symptoms just repeated (desktop reconfigures, screen does not respond)
- rebooting the laptop fixes the problem
Since it works sometimes and works after reboot it seems to me the problem must be a software or firmware one, rather than a hardware issue.
It is a major pain in the neck having to reboot often because of this issue.
NOTE here https://www.dell.com/community/Monitors/U2718Q-MacBook-Pro-won-t-wake-up/td-p/6072760
Dell tech support has basically told Mac users to get stuffed
Perhaps the problem is specific to Dell monitors, I see a few questions about similar issues with Dell + macOS.