I think this one is a head-scratcher: My customer has a Dell Latitude 3540 laptop, with Windows 10 installed, version 1909. All the latest patches have been applied. (I applied them myself a few days previously.)
Two days ago, when she logged in all she could see was her cursor on a completely black screen.
I was able to connect remotely and I saw the same thing. When she pressed CTRL+Alt+Del, we could see the dialogue. If we chose "Change password", we could see the Change Password dialogue. But if we chose Task Manager, we could see nothing. In the end I had her send the machine in to our Help Desk office.
Now that the machine is in our office, I can't reproduce the symptom. Yuck. Even when I log in with her credentials, everything works fine.
What could be causing a completely black screen? And why did it go away?
I suspect this is an example of, if we can reproduce the symptom, we can fix it. For now, I can't reproduce it. It's just embarrassing that I have no idea what happened and now I'll send the machine back without fixing anything.
Note that neither the power button nor a shutdown is sufficient. Since windows 8, the "fast boot" feature makes shutting down fairly effective as a recovery option because the OS starts back into memory from a saved state, with programs and DLLs potentially in an unhealthy state also being restored, whereas restart does a full restart of the OS and reloads the state into memory cleanly.