I have checked and made sure that all the power plugs are tightly plugged in, and that the power is on.
When I press the power on button, the fan in the back of the CPU powers on, as does the fan on the processor. No lights (neither green nor red) adjacent to the power switch go on.
I have tried switching to another hard drive, as I have a spare one sitting inside the machine, but it didn't help.
My computer is a 4-year-old custom-built one. I'm not sure if the problem is with the motherboard (I have read that resetting it might help, but 1) I have no idea how to re-set it (my MB is Fujitsu Siemens D1675), and 2) if the processor fan powers on, does that mean the MB is ok?)