Question:

Larry writes: Occasionally, I will come home to my Mac Pro, which I had set to sleep, and find it powered up but with a black screen. No combination of keystrokes or mouse movements can wake it and my only recourse is to perform a forced shutdown, endangering any data I had opened when the Mac originally went to sleep.

What could be causing this and how can I stop it?

Answer:

There are several troubleshooting steps we can take in situations such as this one. First, let’s look in Console.

The next time you encounter the problem, reboot the Mac and launch Console first. Scroll up to examine the messages that occurred immediately before the startup messages. The easiest way to do this is to look in “All Messages” and scroll up, keeping an eye on the timestamps. Once you find the gap preceding your reboot, stop and examine those messages, looking for clues as to what caused the issue.

The next steps are to perform both a PRAM and SMC reset. For instructions on PRAM resets, see Apple’s support article HT1379. For SMC reset instructions, which vary by machine, see article HT3964. 

Finally, the next time you encounter the problem but before rebooting the computer, see if you can connect to it from another Mac while it’s in its black-screen state. This will require some configuration beforehand.

Using System Preferences, jot down the instructions on how to remotely connect to your Mac. 

Go to System Preferences > Sharing and enable Remote Login. Once enabled, you’ll see instructions on the right for how to connect to your Mac remotely. Write these down for later. Then, once the machine is in its black-screen state, attempt to connect via Terminal using the instructions you wrote down.

If the connection is successful, you can try to look at the recent logs by typing: 

sudo dmesg 

Here, again, look for any clue as to what’s causing the issue. At the very least, this remote connection will give you an opportunity to power down the Mac properly (as opposed to a forced restart). To do this, type:

sudo shutdown -r now

Hopefully, one of these methods will either solve your problem or point you in the right direction.