Turn off your computer.
Turn on the monitor.
As soon as you turn on your computer, your monitor should show something, either a "splash" screen, or some "diagnostic" information (CPU, RAM, detected CD/DVD drive, detected disk-drive, et cetera).
If you are seeing "nothing", then the power-supply could be broken.
When It is working correctly, it produces different voltage-levels for each component.
When it is not working correctly, it may cause some lamps to light-up, or the open/close button on the CD/DVD drive will work, or you might hear the fan(s) inside the computer spinning, but the required power to the "motherboard" is not being sent.
Or, if the power-supply is OK, it could be the CPU or the motherboard that has failed.
The only "good news" is that your personal files, on your disk-drive, are highly-likely to be unaffected.
It's time to take the computer to a qualified computer technician, for proper trouble-shooting.
DO NOT LET THE TECHNICIAN TRY TO REINSTALL WINDOWS - THAT COULD DELETE ALL YOUR PERSONAL FILES.