I’m so confused!!!! No input signal.

I am beyond lost. I have a new motherboard and my moniter when plugged in syas no input signal whether the computer is on or off. I unplugged all drives and removed motherboard from case with only motherboard and power supply attached. Still sayd no input signal. I tested the motherboard at the store with my ram and it works. I tested the moniter and power supply with another computer and they both work. But when I put them all together, no dice. What do I do?