I have the same thing happen occasionally- a complete freeze. The most common and repeatable situation is this:
My SP4 m3 is in landscape orientation and tablet mode. I open Edge and the window is not fully maximized (this is a problem I wrote about in another post).
To maximize the window, I rotate the Surface to portrait orientation and use Action Center to switch off tablet mode.
In desktop mode I attempt to maximize the window, with the intent to then re-enter tablet mode. Usually the device freezes after rotating or upon tapping the window resize icon.
The only way to resume using the device is by the power/volume up button for 15 seconds then power button. Control+alt+delete, escape, or sleep don't work.
I get no error messages, and three or four times I've looked at Event Viewer and there are no events logged other than the shutdown was unexpected.
Edge seems to be the major offender, and this has happened occasionally since I got the Surface new a month or so ago.