|Just to clarify - this is desk top not a laptop; as initially I misread this as a laptop...|
What make and model - whichever it be?
Did you make "any" changes to the system prior to fault arriving?
If a desktop, have you carefully checked "all" on-board connections?
Do you have a voltmeter, and are happy to use it inside the unit?
Is the on/off switch a physically mechanical operation; in that you can feel and maybe even hear it operate - (click) - rather than something akin to a soft contact... Is there anything immediately built onto the rear of the switch...
Daveincaps suggestion immediately above are worth following too; not least the psu incoming mains voltage setting - although I think many these days auto- sense incoming (line voltage) and "adjust to it accordingly"...