Normally a monitor have an indicator light. Is that one on or off? If no light it shows that
1 there is or no power supplied via main cable
2 Main cable defective, try an other one
3 Main switch? Some monitors have on the backsite a main switch and on the front a small on/off switch.
4 all ok above well your build in powersupply ias out of order. The display is not working anymore, so you can at least try to find a little fault, example a fuse on the power supply board or so. Check by metering if supply voltage is available on psu and output voltages on psu etc.
BE CAREFULL FOR LIVE HIGH VOLTAGES FROM MAINS SUPPLY!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!