Computer died please help

i was on my desktop and everything was fine but i came back to my computer from doing something else and it was on standby. i try and revive it and my monitor isnt getting any signal from the computer. This has happened twice before but when i tryed to turn it on there werent any error code beeps all the other times there have been. please help D:

how much do those generally cost, i dont know anything about computer hardware how the fuck do i take it out to see what kind i need, and are you sure its the power supply because the computer still turns on its just nothing happens.