My Monitor turns itself off without a sleep command

I hope someone knows the answer.

I have a Dell Inspiron 3847 running Windows 10.

Several times over the past 6 months I have noticed my computer still on and the Screen has turned off. Neither the Keyboard or the Mouse responds either.

The only way I can switch off is to hold the power button for a few seconds. I have prevoiusly made sure the Sleep options, if ever i use them, is only for the computer and never the screen. Thanks