please tell me why my restart button isn't working!

Because the computer has a restart button I am assuming it is quite old or at least in an old ATX case: usually the cable from the restart button comes from the button itself into the motherboard.

My first step would be to open the case and trace the wires coming from the reset switch, it will lead back to (in most cases) a black 2 point female connector. This connector will fit onto the motherboard where all the other pin connectors go to (will be a block of pins where the other wires go to), you should be able to trace the power switch back to the correct pin block as I'm assuming that is working :)

Read the motherboard manual for the location where the reset switch should be plugged into, sometimes it is even labelled in white PCB transfer on the motherboard.

Make sure it is connected properly to the pins, 9 times out of 10 it is just not correctly connected or at all.