This is the most dreadful screen for most of the Mac users. Some say that it’s almost as bad as the Window’s blue screen.
This screen seems to indicate that something is wrong with your Mac. But don't worry! If nothing is wrong, it's an easy fix.
How to get rid of the error
Typically, to solve this problem, all you have to do is press the power button on your Mac for a few seconds – just like the screen says – until it’s off and then press it again to turn on. That's usually it.
However, this problem might persist. In this case, disconnect all the external devices connected to your Mac and restart. You can leave your mouse connected, though
Another solution is to restart your Mac in Safe Mode. To do that, turn off your Mac by pressing the power button, then turn it back on, press and hold the shift key as soon as you hear the startup sound and release it once the Apple logo pops up. That’s it, if your Mac seems to be working just fine, you can restart it again and log-in normally.