MBP occasionally doesn't wake up until after forced reboot showing a message of an unknown problem
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Troubleshooting is a process of elimination and can take a while to identify the root cause of a problem.

Here are some steps to get you started.

Resetting the SMC 

The steps for resetting the SMC on your particular MacBook are:


Shut down your computer.
Keep the MagSafe adapter (power cable) plugged in.
Press at the same time shiftoptioncontrol (on the left side of the keyboard) and the power button.
Let go.
Turn your computer back on with the power button.


Since this problem seems to be intermittent, it may take a while to know whether the reset has resolved it or not. 

Test your hardware 

Another thing you can also do is test your hardware. Depending on the year of your Retina 13" MacBook, this can be done by using either Apple Diagnostics or Apple Hardware Test. 

One you've checked out the links, follow the steps for the one suited to your model and share your results.