Computer Turns on but no Display on Monitor.

Hello, my computer is annoying me. It switches on, from the box or whatever it's called, but the monitor says no signal. I've made sure that everything is plugged in correctedly, too. The kepyboard doesn't have the lights on, nor the mouse as they're plugged in from the computer. after reseaching on the problem, some say I need a new power supply. Is that true? As when I tryied to do the trick "plug it out, hold button for 30 seconds, plug back in" trick, it still doesn't work. Also, when switching on computer, it doesn't make any beeping sound, if that helps.