Any cut in power to the PC or Controller is disaster while cutting a job due to lost signals causing positional error. Even tiny drop outs will have an effect.

The affect of full power cut will depend on several variables and how the Control system is designed. The type of power supply for the drives, spindle type and settings, feedrate, tool size, material type, depth of cut, Axis/gantry weight etc will all play there part so not something that be predicted accurately.

For instance toroidal PSU stores energy in it's capacitors so drives and motors will remain under power until drained so machine will run on slightly after power drops.
Spindles will run on for so long depending on type, power rating, tooling size etc. For instance VFD's often used for spindles have capacitors which will allow spindle to run on if not commanded to stop.

My advise would be to use UPS that can keep the PC/controller and VFD powered and allow for a Safe E-Stop in the event of power cut. This will mean the Control system will have to be designed in such away that it can detect power loss and shut down important areas quickly and safely.