If a pc makes beeping noises and the monitor isn't working, what's wrong?

ok so i faced a problem with my pc, and its at a workshop, so i got my old DELL Dimension 3100 out of the closet, plugged it in, made sure everything is good, turned it on, as soon as it starts, it makes a beeping sound, and the screen says no signal,?, so i press the power button and it instantly shuts down, i... show more ok so i faced a problem with my pc, and its at a workshop, so i got my old DELL Dimension 3100 out of the closet, plugged it in, made sure everything is good, turned it on, as soon as it starts, it makes a beeping sound, and the screen says no signal,?, so i press the power button and it instantly shuts down, i take off its cover, make sure the ram was in goodly, and i found pretty dusty, so i cleaned it with a duster, didnt use a liquid, same problem, please help, posting this from my friend's laptop.
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