In the left top corner of the screen is a flat wire . Make sure it is properly seated.
It can come lose when the hard drive is replaced, and maybe you did not properly seat it back. The mac starts up, but as soon as the kernel loads it dims the screen, and then turns the screen off.
Interestingly, when I try to reboot the mac by holding the power button pressed, it would flash the screen. Apparently the power button briefly over ride the kernel control of the screen.