Reading, from what you write, I would check the power supply, because it should run the HDD, when you turn on the PC.
One test you can do is to disconnect the motherboard, and leave the power supply connected to the HDD only, if you turn it on and it doesn't turn, it means that one or more voltages are not in place.
Maybe you can check all the voltages of the power supply, but it's not said that if they are there, then by connecting the MB, it works.
AMD 286-16 287-10 4MB HD 45MB VGA 256KB
AMD 386DX-40 Intel 387 8MB HD 81MB VGA 256KB
Cyrix 486DLC-40 IIT387-40 8MB VGA 512KB
AMD 5X86-133 16MB VGA VLB CL5428 2MB and many others
AMD K62+ 550 SOYO 5EMA+ and many others
AST Pentium Pro 200 MHz L2 256KB